South Platte and Metro Basin Hydrology

Hydrology Concepts - The Hydrologic Cycle

Hydrology is the study of water.

As a science, hydrology has evolved in response to the need to understand Earth's complex water systems and help solve water problems. The supply of water available for human use is limited by nature. Although there is plenty of water on Earth, it is not always in the right place at the right time and of the right quality.

Hydrology encompasses the occurrence, distribution, movement and properties of water and its relationship with the environment within each phase of the hydrologic cycle, shown at right. The hydrologic cycle, or water cycle, is a continuous process by which water is purified by evaporation and transported from the Earth's surface (including the oceans) into the atmosphere and back to the land and oceans. All of the physical, chemical and biological processes involving water as it travels its various paths in the atmosphere (e.g., condensation and precipitation), over and beneath the Earth's surface (e.g., infiltration and snowmelt runoff) and through growing plants (e.g., transpiration) are studied.

There are many pathways water may take in its continuous cycle of falling as rainfall or snowfall and returning to the atmosphere. Water may be captured for millions of years in polar ice caps. It may flow to rivers and finally to the sea. It may soak into the soil to be evaporated directly from the soil surface as it dries or be transpired by growing plants. It may percolate through the soil to ground water reservoirs (aquifers) to be stored or it may flow to wells or springs or back to streams by seepage. The cycle for water may be short or it may take millions of years.

~ From the USGS Water Science School.

Hydrology Concepts - Variability

Hydrologic variability occurs across space and time. Within the South Platte Basin, flows can vary between sub-basins within the same year, with one sub-basin experiencing drought and another experiencing adequate water supplies. Variation can occur because of elevation, regional weather patterns, water storage projects, impacts of specific storm events and other reasons.

This interactive visualization shows annual flow volume in acre-feet from 1950 to 2011 for 32 streamgages in the basin, representing the South Platte River and many of its tributaries (Source: StateMod, Natural Flow time series). Streamgages on the South Platte River are ordered from upstream to downstream and tributaries are generally arranged in order from where their confluences are with the South Platte River. Within a streamgage, average volumes are ranked and placed into percentile categories as follows:

• Less than 10^th percentile = Much Below Normal
• 10^th - 24^th percentile = Below Normal
• 25^th - 75^th percentile = Normal
• 76^th - 90^th percentile = Above Normal
• Greater than 90^th percentile = Much Above Normal

Click on the categories within the legend to select/deselect the categories of flow conditions. Note that 2002 was a severe drought across the entire basin and resulted in many changes, including the adoption of water conservation practices. See the Drought section for more information.

It is important to recognize that the data used in an analysis can be impacted by human actions. Streamgage measurements reflect the impacts of diversions, reservoir releases and other impacts. Therefore, evaluating streamgage data also indirectly reflects other conditions in the basin. Annual data can also be impacted by extreme events. For example, a streamgage may indicate drought conditions for the majority of the year and one large rain event may skew the annual result (e.g., the September 2013 flood event centered on the Colorado Front Range). It may be necessary to view data at finer detail, such as a monthly interval, to fully understand conditions. Fully understanding drought conditions requires evaluation of snowpack, precipitation, natural flows, reservoir storage and other data.

Hydrology Concepts - Watersheds

A watershed is an area of land that drains all the streams and rainfall to a common outlet, such as the outflow of a reservoir or any point along a stream channel. A watershed can also be referred to as a drainage basin or catchment; it is also possible to think of a watershed as a precipitation collector. Ridges and hills that separate two watersheds are called the drainage divide. In Colorado, the Continental Divide is an example of a drainage divide at the largest scale. Water that falls west of the Divide will eventually make its way to the Pacific Ocean via rivers like the Colorado River. Water that falls east of the Divide will eventually make its way to the Atlantic Ocean via rivers like the South Platte and Arkansas. Natural drainage features may be modified by human impacts such as dams, tunnels, pumping, canals and other infrastructure.

A watershed consists of surface water--lakes, streams, reservoirs and wetlands--and all the underlying groundwater. Larger watersheds consist of many smaller watersheds. The size of the watershed depends on the outflow point: all of the land that drains water to the outflow point is the watershed for that outflow point (USGS Water Science School). It is helpful to locate streamgages at the outflow point of important water supply watersheds or stream confluences.

There are a number of watershed datasets. A common one is the Watershed Boundary Dataset (WBD) from the U.S. Geological Survey, which is a nationwide dataset. Here, watersheds are called hydrologic units and form a standardized system for organizing, collecting, managing and reporting hydrologic information. Hydrologic units are arranged in a nested, hierarchical system with each unit in the system identified using a unique code. Hydrologic unit codes (HUC) are developed using a progressive two-digit system where each successively smaller areal unit is identified by adding two digits to the identifying code of the parent basin. An example of this hierarchy is shown below.

Source: Watershed Boundary Dataset

The WBD contains eight levels of progressive hydrologic units identified by unique 2- to 16-digit codes. The dataset is complete for the United States to the 12-digit hydrologic unit. The map at right shows HUC10 watersheds in the South Platte Basin.

The State of Colorado classifies watersheds into divisions (the South Platte Basin is Division 01) and districts, which correspond to basins associated with water administration. Water-related data in the State's HydroBase database, such as diversion records and water rights, are assigned a "Water District Identifier" or WDID; the first two digits of the WDID are the water district number and the remaining digits indicate a structure such as a diversion headgate or reservoir. On the map at right, click on the second Layers icon in the upper left corner to toggle to a map of the water districts in the Basin. Note that the Republican River and Laramie River basins are sometimes considered with the South Platte Basin but that each has unique water resources issues.

Hydrology Concepts - Streams

Streams are the surface water conveyance features within a watershed. There are a number of stream datasets, including:

• National Hydrography Dataset (NHD) - from the U.S. Geological Survey.
• Source Water Route Framework (SWRF) - developed by the Division of Water Resources and derived from the NHD. The SWRF represents most streams in Colorado, in particular those with water rights or other important features. However, SWRF may not include minor streams or features that do not have a nexus with water administration.
• Colorado Department of Public Health and Environment's water quality stream segmentation dataset - classifies stream segments based on level of impairment and the reasons for impairment (E. coli, sediment, temperature, etc.).

Similar to NHD, the SWRF uses a Geographic Names Information System (GNIS) ID, an 8-digit identifier used to uniquely identify most streams in the state, rather than using the stream's common name (there are 13 streams named Dry Creek in the South Platte basin alone). SWRF provides a more simple dataset than NHD: in the SWRF each line represents a stream or river, whereas NHD provides many individual line segments for each stream or river. It is important when using these datasets to indicate the version because government agencies do periodically adjust the data.

By using the NHD linked to the SWRF, it is possible to link federal and State datasets. Datasets such as stream gages, diversion structures and instream flow rights (discussed in subsequent pages in this story) can be "referenced" to the SWRF to provide a common framework for analysis; such data work is ongoing in order to leverage the SWRF dataset.

This map shows a small section of the SWRF, representing the southwest portion of the Basin. The SWRF layer shown here is cropped because showing the full basin would result in a slower visualization. The full SWRF can be downloaded here.

Hydrology Concepts - Water Resources System

In addition to the natural hydrologic system, water resources in Colorado include human-created systems containing the following constructs:

• legal constructs, such as water law
• physical constructs, such as infrastructure that is used to store and convey water
• operational constructs, such as agreements that control how systems are operated to deliver water to paying customers or shareholders

Surface water use in Colorado is administered by the Division of Water Resources (DWR). DWR is tasked with providing the dependable distribution of water in accordance with statutes, decrees and interstate compacts. DWR relies on real-time and other data from its own streamgages, as well as data from federal agencies such as U.S. Geological Survey (USGS), Bureau of Reclamation, Natural Resources Conservation Service (NRCS) and local entities such as cities and water districts.

A diagram representation of a water system can represent the complexities of the system in a more readable format, although the information is complex. The complexity of water resources systems is illustrated by “straight line diagrams” that combine physical and legal information to help people understand water resources systems. The diagrams are graphical linear representations of structures and water rights for a given stream system that allows a large amount of complex information to be observed at a glance. This straight line diagram of water district 04 (Big Thompson River) can also be viewed and zoomed into here. Straight line diagrams for other basins are also available.

Hydrology Concepts - Return Flows

It may seem as though water is continuously removed from streams for agricultural, municipal and other uses and therefore the amount of water in a stream decreases as one moves downstream. However, water that is diverted for municipal, industrial or agricultural purposes that is not consumed returns to rivers and/or aquifers by surface flows or underground flows. For example, the U.S. Geological Survey has estimated that approximately only 37% of the water diverted statewide for agriculture is actually consumed (Citizen's Guide to Where Your Water Comes From, p.12).

These surface and underground flows are collectively called return flows and can be used by downstream water users. Types of return flows include runoff and infiltration from agricultural lands, runoff and infiltration from lawns and landscaping and returns from wastewater treatment plants or industry. Irrigation return flows, in particular, are often reused multiple times. Unless a municipality relies exclusively on groundwater or its water supply comes directly from a river’s headwaters, its supply includes reused water. The term “direct reuse” indicates wastewater that is directly used for municipal supply and “indirect reuse” indicates mixing wastewater with other supplies. Many cities in Colorado rely on reused (or recycled) water (Citizen's Guide to Where Your Water Comes From, p.24; Citizen's Guide to Colorado Water Conservation, p.20-21).

Water that has been diverted from other basins (transbasin diversions) contributes flows to streams and can result in some streams actually being larger than natural conditions.

This diagram, sometimes called the "snake diagram", from DWR shows how flows greatly increase downstream of Denver, which allows reuse for agriculture and other purposes. In general, municipalities seek cleaner water supplies, which often means acquiring mountain supplies. In some cases, exchanges can occur, for example, allowing a municipality to use cleaner mountain water supplies in exchange for agricultural or other users taking water from municipal water rights downstream. This allows different water providers in a region to share infrastructure and water supplies to optimize the system.

Notice how both the South Platte and Arkansas rivers do not gain large amounts of flow like rivers on the West Slope. Rather, these rivers' widths are narrower and a more consistent width along their lengths. This is an indication that the basins rely on return flows and reuse to utilize lower basin supplies to meet multiple needs.

Resources for understanding more about return flows in the Basin can be found here. The report "Return of Seepage Water to the Lower South Platte River in Colorado"(7.6MB) also provides historical context.

Hydrology Concepts - Natural, Regulated and Available Flows

Streamflows can represent various conditions. Regulated flow is streamflow that is impacted by humans and corresponds to streamgage measurements that are commonly reported.

Natural flow is streamflow absent the impacts of humans (no reservoirs, no diversions, etc.). Natural streamflow can only be measured where human impacts are absent, such as headwater basins. Natural flow can be estimated from regulated flow by removing human impacts. For example, natural flow can be estimated by subtracting reservoir releases and adding back in diverted flows from ditches and canals. The calculations can be more complex if all hydrologic inputs are considered, such as return flows, evaporation from reservoirs, losses to groundwater aquifers, etc.

Available flow is water that has not been allocated at a point in time and space. For example, high flows in spring may provide enough water to meet all water demands considering water law and excess water is available for additional use. This condition is called “free river” and means that there will be no calls on the river to curtail junior water rights. Available flow may be available at any time when the river can meet all water demands. In a planning study, the amount of available flow indicates the opportunity for additional water development, such as building a reservoir to store the available flow, or implementing an instream flow right to preserve environmental flows.

Hydrographs (see the Hydrographs page of this story), can be used for natural, regulated or available flows, as well as any other subset or aggregation of flow data. The interactive hydrographs at right are for two locations on the South Platte River: higher up in the basin near Elevenmile Canyon Reservoir and near the stateline at Julesburg. Depending on the location and impacts of regulation, natural flows may be larger or smaller than regulated flows. Available flow will accumulate as one moves downstream on a river. Julesburg is near the State line and therefore indicates water that has not been fully utilized in Colorado and is flowing into Nebraska. Agreements between states, called “compacts”, define how much water Colorado must deliver to other states. See the South Platte River Compact.

Analyses of water supplies and the effects of climate change on streamflow are a couple of ways where it may be necessary to estimate natural flows. Estimation of natural flows and understanding conditions in a system typically require utilizing models. Two examples are point flow models and the CDSS StateMod model, which will be discussed in the next section. Additionally, the USGS's StreamStats application provides estimates of streamflow statistics for ungaged (natural) sites.

Water Demands - Agriculture

Understanding current and future available water supplies requires understanding the demands that are placed on those supplies. Various entities compete for the Basin's water resources and these entities need water at different times of the year. Agriculture is the largest user of the Basin's water resources, accounting for 85% of total water diversions (Colorado Water Plan, p.3-13).

Agricultural demand for water is typically from May to October, although there is demand in the colder months as irrigation reservoirs are filled to prepare for the following year. Crop irrigation requirements are defined as the depth of water needed to meet the water consumed through evapotranspiration by a disease-free crop, growing in large fields under non-restricting soil conditions including soil water and fertility, and achieving full production potential under the given growing environment. Irrigation requirements vary depending on farming practices, irrigation method and the crops that are grown. Crop water demands at a location will be reduced by local precipitation. However, rain in Colorado is highly variable and does not necessarily occur at the needed location and time. Consequently irrigation is often required.

This visualization shows the irrigation water requirements for crops grown in select locations in eastern Colorado. In general, alfalfa requires the most water, followed by sugar beets (Source: CSU Extension Fact Sheet: Seasonal Water Needs and Opportunities for Limited Irrigation for Colorado Crops).

Some additional considerations are:

• Crops need water more at specific times, such as when corn tassels appear, in order to ensure maximum yield.
• Grains generally have more flexibility in irrigation, whereas high-value vegetables have less flexibility in irrigation, hence the value of wells, which can be used at any time if water can be taken.
• Crop growing seasons are also impacted by weather, such as freezing temperatures and hail, and field conditions that impact the ability to use farm equipment, which may impact when irrigation water is applied.
• Water deliveries to farms are managed by ditch and reservoir companies, typically in a way that simplifies and optimizes operations, such as rotating deliveries to laterals within a ditch system.

Water Demands - Agriculture

Agricultural producers rely on various data to help make decisions about the upcoming irrigation season, including reservoir storage conditions, snowpack conditions, water rental programs and other information. Water use for each agricultural producer and ditch system depends on the location, conditions and operations of the system, as well as the seniority of their water rights.

When water is diverted from the river to agricultural land, it is done via a system of canals and/or ditches. These systems are not 100 percent efficient, meaning that some water is lost during conveyance. Because of this, there is a need to divert more water from the river than is necessary for crop consumption. Water lost during conveyance, however, is not necessarily truly lost, as it returns to the groundwater and ultimately to the river as return flows for use by downstream water users. Similarly, the amount of return flow varies by the method of irrigation (flood vs. sprinkler vs. drip). Changing irrigation methods has impacts on downstream users and the environment.

This interactive visualization shows diversion records for five ditches in the Basin and illustrates some conditions and circumstances under which ditches divert water for agricultural purposes. To select/deselect a ditch on the plot, click on the ditch in the legend at the bottom.

• Brantner Ditch: this is an example of a ditch with senior water rights that is able to support the same use nearly every year because of those senior rights. To clearly see this, click on the other ditch names in the legend to "turn off" those ditches so that only Brantner Ditch is shown. Also note that the ditch starts diverting water around the first of April each year.
• Riverside Canal: this is an example of a ditch that supplements its supply with reservoir storage. This type of ditch can irrigate at "normal" levels, even in drought.
• Harmony Ditch: this example ditch highlights innovation. This ditch has junior water rights, yet has changed irrigation methods (flood to sprinkler) and the type of crops grown to increase the certainty of producing crops even when water short.
• Big Thompson Ditch & Manufacturing Company: this is an example of a ditch that has rights to Colorado-Big Thompson (transbasin) water and no independent, off-channel storage. In general, diversions decline as the season progresses, which can be due to a number of reasons, including the types of crops that are grown.
• S. Boulder Canon Ditch: this is an example of a ditch that frequently experiences shortages. Not only does this ditch divert less water than the other four ditches shown, but it diverts over a shorter time period. Each year, the ditch diverts water for about two months.

Water rights that are used for irrigated agriculture can be transferred to other uses such as municipal use. In this case, only the consumptive use portion of the decree can be transferred to the new use and average historical return flows must be maintained to minimize impacts to downstream water users. Estimating the historical consumptive use based on historical weather data, crops and irrigation practices requires analysis, such as the CDSS StateCU software. The value of agricultural water that might be transferred to a municipality depends on its water right priority date, yield during high demand times of the year and ability to deliver water to the water treatment plant or other location.

Water Demands - Municipalities and Industry

Municipalities and industry are a second category of demand placed on the Basin's water resources. Municipal and industrial water demand tends to be fairly predictable from year to year and season to season in a general sense, although variation does occur. Municipal water demand is fairly constant throughout the year for indoor use but greatly increases for outdoor use in the summer months to water landscaping. Variation from year to year depends on weather and over time is impacted by long-term drivers. Variation within a year depends on weather and specific events.

Water supply planning is typically framed by short-term planning (which considers system water supply firm yield and storage, infrastructure projects that might disrupt supply, short-term disasters, etc.) and long-term planning (which considers population growth, changes in efficiency, climate change, etc.). Planning involves understanding risk (quantifying loss) and uncertainty (recognizing multiple possible outcomes). Drought planning typically takes into consideration that a system has the ability to supply water for a certain amount of time to meet normal demands and that extreme conditions, if severe enough, will result in changes such as water restrictions. For example, a utility may be able to provide normal water deliveries for three years even if a 1-in-50-year drought is occurring. A normal level of operations allows customers to behave normally and ensures a normal level of revenue for the utility.

System reliability and long-term planning typically involve analysis and modeling where multiple scenarios are used as input to understand potential impacts. For example, the worst historical drought (based on low streamflows, snowpack, etc.) may be used as input to a model of the current system to determine whether additional water supplies or changes to operations are needed. Based on the analysis, a water provider such as a utility may decide to acquire additional water supplies as an insurance policy to decrease the certainty of a negative impact, thus reducing risk. Each water provider must perform their own analysis based on their water supply portfolio and local conditions. In Colorado, the worst situation is when a statewide drought occurs that impacts many basins.

The interactive visualization at right shows the total water demands of Denver, Aurora and Boulder. The hydrographs (from StateMod, Total Demand time series) clearly show that water demand almost triples in the summer months for these large municipalities compared to the winter months. Water use data are often provided in municipal water efficiency plans or annual reports. Denver, Aurora and Boulder all have water efficiency plans.

The water depletion from the natural system to meet municipal demand is not fully consumed. Much of the water used for indoor purposes will be returned to streams as treated wastewater effluent and a portion of outdoor irrigation will return to the surface and groundwater system as return flows. Municipalities and industry focus on the first delivery of water to customers, which drives the need to ensure supplies. The term “water use” can therefore be confusing depending on whether demand, depletion or consumptive use are being discussed.

Water Demands - Environment and Recreation

Environmental and recreational needs are a third category of demand placed on the Basin's water resources, but in this case, the demand is to keep water in the river and other water bodies. The South Platte Basin Implementation Plan (BIP) (p.2-25) describes the environmental and recreational attributes that are important to the Basin. These include the protection of habitat for fish species like greenback cutthroat trout and plains fish species like plains minnow, common shiner and stonecat, to name a few. Non-fish species that are considered important include species like boreal toad, yellow mud turtle and river otter. Recreational activities that are considered important to the Basin include fishing and both flatwater and whitewater boating. Protection of the riparian plant community is also important to the Basin. Protection and enhancement of the environment has a direct economic impact as well as contributing to the characteristics of Colorado that residents value and expect.

The protection of these varied environmental and recreational attributes corresponds to the protection of flows that meet these attributes' needs. For example, many fish species require a range of flows to thrive. Flushing (high) flows move sediment downstream, create new habitat and can be cues to spawn. The life stage of the species (larval, juvenile or adult) can also determine the necessary flows. All species need some minimum flow that will ensure they have adequate habitat to survive. Minimum flows are the smallest amounts of flow that will maintain hydraulic parameters for fish habitat, such as specific velocities and water depths. Species can survive low flows and variation in flows but can be severely impacted by conditions in which the magnitude, timing, and/or persistence of flows are outside of biologically-tuned ranges. It is a challenge to quantify required flows for each species and therefore indicator species and/or habitats are often used to monitor and evaluate habitats.

For a given stream reach, the flows necessary to protect environmental and recreational uses can be highly varied and also vary within a year. This interactive visualization shows the flows needed for some environmental and recreational attributes on the South Platte River below Chatfield Reservoir, as well as the streamflow that was actually present during that time. (Source: South Platte BIP Appendix D-2 - Environmental and Recreational Assessment Methodology and Framework). This visualization, while just an example, shows that it can be challenging to meet environmental and recreational flow recommendations in any given year. There are opportunities to meet flow recommendations by reducing demands, managing reservoir storage releases and water deliveries through a reach and changing operations.

The Environmental and Recreational component of the Statewide Water Supply Initiative (SWSI) Update is focusing on the development of a Flow Tool, along with an update to the Environmental and Recreational database. This tool will be able to assess flow conditions in each basin and help basin roundtables assess their environmental and recreational flow needs and if those flow needs are being met.

Meeting Demands through Water Management and Administration

Since numerous demands are placed on Colorado's water resources, it is necessary to have a process by which water is administered, managed and regulated. A legal framework called the prior appropriation doctrine regulates the use of surface water in Colorado, as well as tributary groundwater that is connected to streams. This doctrine is also referred to by the phrase "first in time, first in right". This means that in times of short water supply, court-decreed water rights holders who obtained their rights earlier (senior rights) can use water before decreed rights with later dates (junior rights) may use any remaining water. The water user must have a definite plan to divert, store or otherwise capture, possess and control water, and must specify the place of diversion or storage, amount of water, type of use and place of use.

The Colorado Division of Water Resources, which includes the State Engineer, division engineers and water commissioners, has the authority to administer water rights. In particular, water commissioners have the primary task of going into the field and distributing water in priority and according to decreed terms. This involves responding to calls for water, monitoring headgates, issuing orders to reduce or cease diversions and collecting data on diversions. All of this data is collected on a real-time basis via a statewide satellite-linked monitoring system. Here is an example of how a call for water works and how the water commissioner administers the call:

1. After peak streamflow has been reached due to snowmelt, streamflows start to drop. Some water users in a river may not have sufficient water to fulfill their court-decreed diversion amount. One farmer notices that she does not have enough water to irrigate her farm and places a call to the local ditch official.
2. The farmer can only call for the amount of water provided in her water right decree and only for the amount that she can actually put to beneficial use (e.g., to irrigate her fields).
3. The ditch official contacts the water commissioner and places the call. A verbal call can be made but often a formal, written call is required.
4. The water commissioner verifies that the call is legitimate and then starts looking upstream to shut down all undecreed uses. After this, it is determined that this is not enough water for the farmer.
5. The water commissioner then limits all decreed upstream users to their decreed amounts of diversion. Yet this is still not enough water for the farmer.
6. The water commissioner now uses the "first in time, first in right" priority system to look upstream from the farmer's headgate diversion for decreed users with priority dates junior to (more recent than) the farmer's date. These junior users' diversions will be reduced or shut down.
7. Each decreed junior water user, based on their order of priority, is curtailed until the farmer gets enough water to fulfill her right.
8. Now the farmer has enough water. However, as streamflows have continued to drop, a municipality downstream of the farmer does not have enough water to fulfill its right, which is senior to the farmer's right. The municipality places a call.
9. The water commissioner goes through the same process, reducing or shutting down all rights that are junior to the municipality's rights until the municipality's rights are met. This can mean that the farmer may now have to let water flow past her headgate to fulfill the municipality's senior downstream right.
10. To add more complexity to the process, the priority date of the river call may change daily depending on the streamflow available and the seniority of the diversions that need water on that day.

A water exchange can occur within the prior appropriation system. An exchange allows an upstream diverter to take water that a downstream diverter would otherwise receive, as long as the water is replaced at the same time, place, quantity and quality that the downstream user experienced before the exchange. One consideration is that water exchanges may decrease streamflows in particular stream segments in return for substituting water into stream segments above the water right to which the exchange is made.

The information on this page is derived from the Citizen's Guide to Colorado Water Law. The video at right explains some of the South Platte Basin's water history. (Source: Noah Besser Animations)

Storage - Reservoirs

As has been shown, various users of the Basin's water resources need water at different times of the year and in different amounts. If left to its own means, the snowpack that begins to melt in April would run off quickly into Colorado's rivers and out of the state. Dry conditions during the most of the year produce little water in rivers during the summer and winter months. Historically, the South Platte River naturally dried up for extended stretches during the late summer and early autumn months (Citizen's Guide to Where Your Water Comes From, p.26). Reservoirs are constructed to maintain a consistent supply of water for municipal, industrial, agricultural and sometimes environmental and recreational uses. Reservoirs essentially regulate river flows: they can increase low flows during dry conditions and reduce flooding flows.

Some reservoirs are operated primarily for drought reserve and can remain substantially full until needed. One example is Eleven Mile Canyon Reservoir. This is the second-largest reservoir in Denver Water's collection system. Note how the reservoir is at or above capacity most of the time. Also note how the drought of 2002 affected the reservoir. Over half of its capacity was used in the later half of 2002 and it took until July 2005 for the reservoir to refill. Denver Water publishes reservoir level data on a daily basis.

Other reservoirs are operated primarily for irrigation and tend to fill in non-irrigation months and drain each year during the irrigation season. An example of this type of reservoir is North Sterling Reservoir, an off-channel reservoir. Reservoir levels typically peak in the spring and then decline as the irrigation season progresses.

Some reservoirs may also provide flood storage, meaning that the reservoir is kept partially empty in order to store flood flows. An example of this type of reservoir is Chatfield Reservoir. Chatfield Reservoir was built in response to flooding in 1965 and has the ability to store floodwaters from the South Platte River and Plum Creek. The Chatfield Storage Reallocation Project is currently being undertaken to reallocate uses of the water. It was determined that the reservoir can accommodate an additional 20,600 acre-feet of water storage for water supply without compromising its flood control function. This additional storage space will be used by municipal and agricultural water users.

Carter Lake is an example of a transbasin reservoir. It is a component of the Colorado-Big Thompson Project (see the Transbasin Diversions page). Several cities, including Longmont, Boulder and Broomfield, receive water from Carter Lake. This water also irrigates farms and ranches in Boulder and Weld counties (Source: Northern Water). A condition for transbasin diversions may be the construction of a “compensatory storage” reservoir that benefits the originating basin, in which case storage is used to provide supply that might originally have come from the natural river. Transbasin diversions often also include storage in the receiving basin in order to re-time flows to meet demands over an entire year.

Reservoirs also provide operational flexibility. For example, a reservoir may release water to meet return flow obligations directly (rather than delayed returns from recharge ponds). Systems that include multiple reservoirs can support exchanges where releases from an upstream reservoir can be provided to an upstream entity in exchange for releases from a downstream reservoir to meet the demands of a downstream entity, although this practice can deplete the intervening reach. Conversely, an upstream reservoir may release water to a downstream reservoir while improving flows in the intervening reach. Reservoirs may include gravel pits. Consequently, an entity such as municipal utility may own storage reservoirs that are below the intakes to their water treatment plants, in order to provide options within the region.

Reservoirs may be constructed on channel or off channel. Resistance to on-channel reservoirs has resulted in new storage projects often being designed as off-channel locations or the expansion of existing on-channel reservoirs. Off-channel reservoirs may require pumping. New reservoirs often include hydropower generation in order to generate electricity and revenue.

New reservoirs and changes to existing reservoirs often seek to meet multiple purposes in order to maximize the benefits of storage. The South Platte Storage Study, completed in 2017, identifies multi-purpose water storage options along the lower South Platte River to capture flows leaving Colorado in excess of the minimum legally required amounts. Water storage possibilities include new reservoirs, the enlargement/rehabilitation of existing reservoirs and alternative storage mechanisms, such as underground storage. Adding significant storage to provide environmental flows is a challenge because funding is often not available to pay for environmental water. The nonprofit Colorado Water Trust is one organization that helps improve flow conditions within the water rights system.

Reservoirs do have issues, including concerns about depleting river flows in order to fill reservoirs, losses due to evaporation from reservoir surfaces, climate impacts of energy use to pump water into storage, changes in water quality and temperature and other issues. These factors and others are considered during the permitting process for a new reservoir.

Storage - Groundwater

Water can also be stored underground in aquifers. One benefit to underground storage is that it can have less of an environmental impact compared to constructing a new surface reservoir. Underground reservoirs also experience little or no evaporation, although plants with long roots can extract water from underground storage.

As mentioned in the Hydrology Concepts - Groundwater page, the South Platte Alluvial Aquifer is one of the largest and most utilized alluvial aquifers in Colorado. The USGS estimates this aquifer stores approximately 8.3 million acre-feet of water (Citizen's Guide to Where Your Water Comes From, p.16). The relationship between surface and ground water is managed according to Colorado water law. Most well water rights are junior to surface water rights. Agricultural producers may prefer to use wells because water can be pumped when needed and may be of higher quality than surface water. In order to pump out of priority, a well must be part of an augmentation plan that replaces the out-of-priority depletion and its impact on the river with some other source, such as releases from a reservoir. The pumping impact on the river is not instantaneous and may, in fact, need to be paid back over many years as part of the augmentation plan. Tracking augmentation plans within a system is one of the costs borne by water providers and their customers, as well as DWR staff.

Denver Water is undertaking an Aquifer Storage and Recovery (ASR) study to determine locations where it can potentially inject treated drinking water into underlying aquifers for use in the future. During periods of abundant water supply, Denver Water would take drinking water from water mains and inject it into wells in the Denver Basin aquifer system. The same wells would then pump the water back up from the aquifers when drier conditions are present.

This short video from Denver Water describes more about its ASR project. More information can also be found at Denver Water's website.

Other water providers that use or are experimenting with aquifer storage include Centennial Water and Sanitation District, Town of Castle Rock and East Cherry Creek Valley Water and Sanitation District.

Transbasin Diversions

Transbasin diversions divert water across watershed boundaries, taking water from one stream or river and conveying it into an entirely different watershed. The largest transbasin diversions in Colorado divert water from the west side of the Continental Divide to the east side. Transbasin diversions have been constructed because while 80% of the State's precipitation falls on the West Slope of the Continental Divide, about 90% of the State's population resides in and about 75% of irrigated acreage occurs in the East Slope (Citizen's Guide to Colorado's Transbasin Diversions). In essence, water supply and demand are on opposite sides of the State.

Of the 44 transbasin diversions in the State, 17 divert water into the South Platte Basin. The largest, and likely most familiar, diversions are the Colorado-Big Thompson Project (Northern Water), Roberts Tunnel Collection System (Denver Water) and the Moffat Collection System (Denver Water). Many municipal water providers in the Basin rely on imported water. For example, the Colorado-Big Thompson Project delivers water to 33 municipal entities, including Fort Collins, Boulder and Longmont.

The table below lists the average annual amount of water that is diverted by each transbasin diversion, in acre-feet.

Source: Colorado's Decision Support Systems, HydroBase data with calculations by Open Water Foundation. Also see the Open Water Foundation's transbasin diversion dataset.

The Citizen's Guide to Colorado's Transbasin Diversions provides much more information about transbasin diversions in Colorado.

Drought

Drought is a shortage of water associated with a lack of precipitation. It occurs when the normal amount of moisture required to satisfy an area’s usual water consumption is unavailable. Drought can appear slowly and last for many years or it can be a short-lived "flash drought" event. It can occur locally, regionally or statewide. Drought is a regular feature of Colorado's climate but can be very destructive without adequate planning, response and mitigation. Planning is necessary so that when a drought-induced water supply shortage exists, water providers can take actions to lessen the impacts.

The historic 2002 drought was the impetus for the creation of basin roundtables, the Interbasin Compact Committee (IBCC), the Statewide Water Supply Initiative and municipal water efficiency plans.

Many resources exist to assist water providers/users with drought planning. Primarily, users need to know when drought is approaching, typically by understanding the current year's conditions in the context of historical years and by understanding the severity of an ongoing drought with respect to a local system's water demand and supply. The Colorado Water Conservation Board maintains a Drought Response Portal, shown at right, that provides an overall indication of drought status, as well as links to other resources such as a monthly drought update.

See the Hydrology Tools - Drought Monitor page for information about another drought tool.

Hydrology Tools - CDSS SNODAS Tools

The CDSS SNODAS Tools, developed for the Colorado Water Conservation Board, provide access to a historical archive of Snow Data Assimilation System (SNODAS) data products for Colorado water supply basins. SNODAS data are available for the entire state and are calculated from remote-sensed datasets using a grid across Colorado. Consequently, SNODAS snowpack data are available at locations where SNOTEL is not available. SNODAS data from the National Operational Hydrologic Remote Sensing Center (NOHRSC) are processed daily to calculate Snow Water Equivalent (SWE) and Snow Coverage statistics for water supply basins in Colorado. Snow Water Equivalent is the estimate of the depth of liquid water contained within the snowpack. Snow coverage is a percent of the basin land surface covered by snow (water bodies in the basin are ignored). Mean SWE is displayed in the map using a legend similar to the National Weather Service. Acre-feet estimates of available SWE for local basins and cumulative upstream basins can be used by water supply planners to better understand water supply conditions.

Instructions for using this SNODAS tool can be found under the "About" tab in the upper left corner.

Hydrology Tools - Surface Water Supply Index (SWSI)

The Surface Water Supply Index(SWSI, not to be confused with the Statewide Water Supply Initiative) developed by the Division of Water Resources based on Natural Resources Conservation Service (NRCS) methodology is used as an indicator of mountain-based water supply conditions in the major river basins and sub-basins of the state. The SWSI compares the total volume of water in a basin or sub-basin against the volume available in the same month of historical years. Depending on the month, the volume is a combination of streamflow, streamflow forecast and reservoir storage.

To view the live SWSI map layer in the CDSS Map Viewer, click here. Check the "Surface Water Supply Index" layer. Expand the layer by clicking on the + on the left of the layer name. Then click on the legend icon to see the categories of conditions.