Integrated and Sustainable Water Management Plan for a Growing City - A Case Study (Part 2)
This three-part blog series is derived from an accepted paper that was published as part of the proceedings of the 2020 World Environmental and Water Resources Congress, originally scheduled for May 17-21 in Henderson, Nevada, sponsored by the Environmental and Water Resources Institute (EWRI) of the American Society of Civil Engineers (ASCE). The event was cancelled due to the COVID-19 pandemic.
Part 1 of this series provided background information on the discussions and decisions that were made leading up to this unique and progressive water management plan.
Part 2 (below) provides location information, river hydrology and water rights information, and a detailed description of the proposed water supply plan.
Our next blog (Part 3), the last in this series, will describe the point-flow and exchange model created by Lytle Water Solutions (LWS).
PROJECT LOCATION
PWSD is located in the southeastern Denver, Colorado metropolitan area, while PWSD’s Logan County farms and the LSPWCD service area are located approximately 75 miles northeast on Colorado’s eastern plains (Figure 2). To effectively integrate a future water supply plan that has such an expansive area to be served for multiple uses, it requires the ability to capture and then move water from the places of supply to the places of demand, both for agricultural uses and municipal uses. To understand how that can be accomplished, it is vital to understand the hydrology of the South Platte River in the area that will be the source of the supplies used for this project, including legal restraints on the diversion and use of physically-available water.
Figure 2: LSPWCD-PWSD service area boundaries.
Figure 3: Monthly flow at Balzac Gage on South Platte River
SOUTH PLATTE RIVER HYDROLOGY AND WATER RIGHTS ADMINISTRATION
Flows in the South Platte River in the project area (Figure 2) are highly variable, with seasonally-high flows associated with spring runoff from the Rocky Mountains to the west, then returning to a base flow condition for much of the remainder of the year.
Flows during base-flow periods can also fluctuate significantly due to local or regional storm events. A hydrograph of flows on the South Platte River at the Balzac gage near Brush, Colorado for the period 1999 to 2019 is shown in Figure 3. This is a typical hydrograph showing the variability in South Platte River flows both seasonally and year-to-year. As this hydrograph shows, because of large, short-term changes in flow, understanding daily fluctuations in flow is essential to understanding the potential water supply availability for a new water project.
One significant factor regarding flows in the Lower South Platte River relates to both a quantity and quality issue. A principal reason for the relatively consistent base flow in the river is that this area is downstream from major irrigation areas along the river, providing year-round irrigation return flows. While this provides a more reliable base flow than further upstream on the South Platte River, it also causes water quality issues related to total dissolved solids (TDS) in the water that makes the water non-potable without treatment. The lower quality water is due to the use and reuse of water for irrigation, accumulating dissolved solids from fertilizers and leaching of minerals from soils.
To complicate the highly-variable South Platte River native flow, these native flows are also significantly altered, and reduced, by irrigation ditches along the river that divert water during the irrigation season for agricultural purposes, as well as reservoir diversions that occur principally in the non-irrigation season to fill storage for the subsequent irrigation season. Figure 4 shows the numerous diversion points on the South Platte River in the reach that this proposed water supply project will operate in that have to be accounted for in evaluating flow availability.
If that doesn’t complicate matters enough, physical water supply availability, and its spatial and temporal variations, is not the only criterion that has to be evaluated in terms of water supply availability for a new water supply project. All surface water diversions in Colorado are governed by the Doctrine of Prior Appropriation (“the Doctrine”), which is essentially defined as water being allocated on a “first in time, first in right” basis. In other words, the first user on the stream has to have its right fully satisfied before the next right can take water, and so on until there isn’t any more water to allocate. Therefore, all of the ditches and reservoirs that divert water in the project reach, and downstream of the project reach, including an interstate Compact with Nebraska, have to be satisfied before a more “junior” right associated with this project can divert water.
The terminology related to the Doctrine is the oldest decreed right which gets to divert water first until it is fully satisfied is considered a “senior” right, while rights that were decreed after the oldest right get to divert in order of their “priority date” when there is sufficient water to satisfy those rights. When the water supply is limited, such that not all decreed rights can be satisfied, the rights that cannot be satisfied (i.e., junior rights) are considered to be “out of priority” and the senior rights can place a “call” on the river that requires Colorado Division of Water Resources (CDWR) officials to shut off enough junior rights to assure that the senior rights are satisfied.
Any right that is out of priority is not allowed to divert from the river. The CDWR administers the river on a daily, or more often, basis to determine what rights are “in priority” and can divert, and what rights are “out of priority” and cannot divert water from the river. Therefore, it becomes extremely important to consider the CDWR’s daily administration of water rights and superimpose that legal restraint on the physical availability of water to understand when, and how much, water is available from the river on a daily basis.
Figure 4: Ditch Diversion Locations
Figure 5: PWSD/LSPWCD Ditch Diversion Locations
Given that there is frequently not enough physically- and legally-available water in the river in the proposed project area to satisfy all decreed rights, this proposed agricultural/municipal water supply plan required innovative thinking regarding how water can be captured when it is available, moved and stored to provide reliable and renewable water supplies, and then delivered to points of use to increase the beneficial use of these supplies, all the while avoiding negative impacts on the agricultural communities, i.e., no permanent buy and dry of irrigated lands.
The development of the components of this water supply plan are described in the following sections, as well as the daily model that was developed to efficiently evaluate the project components, so that not only can the potential yield of the project be evaluated but also the economic feasibility of the project.
PROPOSED WATER SUPPLY PLAN
The proposed PWSD/LSPWCD water supply plan has a number of components because of the large area to be served by this project and the yield that is desired to be produced by the project, as well as giving due consideration to the economic viability of the plan related to the infrastructure that is needed to provide water supplies to a number of disparate locations in varying amounts and timing. The components of this project include river diversions, off-stream reservoir storage, river exchanges, groundwater recharge structures, and pumping and pipeline conveyances, as described in the following sections.
4.1 Equalization Storage
The downstream end of the project is the area near Iliff where the PWSD farms are located (Figure 4). PWSD currently owns 13 farms in Logan County that have been in continuous agricultural production for decades, including since 2001 when PWSD purchased these farms. PWSD has both senior irrigation water rights and junior water rights that can be used for multiple purposes, including municipal use and the ability to exchange the water upstream. Initially, the junior water rights will be used to support this project as they are currently decreed for the uses associated with this project. PWSD’s junior water rights can be diverted at two existing ditch locations, the Iliff & Platte Valley Canal and the Powell & Blair Ditch (Figure 5).
Since these junior water rights are not reliable on a consistent basis and the water use is desired to be further upstream to increase its beneficial use, there is the need to be able to store water to make the water more reliable. The proposed Iliff Reservoir, which is considered equalization storage as it is only designed to hold water until it can either be exchanged upstream or discharged, can be filled from the Iliff & Platte Valley Canal and/or the Powell & Blair Ditch at a rate of up to 40 cubic feet per second (cfs). The Iliff Reservoir will be constructed as an above-ground berm to provide adequate storage without intercepting the local groundwater table. The reservoir will be lined to prevent seepage, as it will be constructed in the South Platte River alluvium.
Water stored at the Iliff Reservoir is planned to either be legally exchanged back upstream to Prewitt Reservoir (Figures 4 and 5) (see Section 4.2, below), which then will ultimately provide water to PWSD for municipal use and/or to LSPWCD upstream users. Water can also be discharged from the Iliff Reservoir for delivery back to the river to provide LSPWCD ditches downstream of the Iliff Reservoir for both direct agricultural use and augmentation of well depletions. In this way, the Iliff Reservoir will use existing ditch diversions to provide water into storage at the new reservoir during times of available water even if there isn’t a contemporaneous demand for the water. This storage will facilitate the efficiency of the beneficial use of water by capturing otherwise excess water and subsequently making it available at later times for both municipal and agricultural uses. One of the ways to accomplish the time shift from water supply availability to water demand is the use of river exchanges to upstream storage and use.
4.2 River Exchange
A legal exchange of water in Colorado is when water is introduced into a stream at a downstream location and a like amount is extracted from an upstream location on the stream (“exchange reach”). Exchanges can obviate the need for pipelines to move water upstream to a point of use; however, exchanges can only occur if all of the water rights in the intervening reach from the “exchange-from” point, i.e., Iliff Reservoir, to the “exchange-to” point, i.e., Prewitt Reservoir (Figure 4), are satisfied. Given the variability in flow in the South Platte River (Figure 3) and the numerous senior ditch diversions in this reach of the river (Figure 4), the evaluation of “exchange potential”, i.e., when an exchange can occur, is very complex.
In evaluating exchanges, it is necessary to understand the native flow of the river, the diversions by senior water rights in the exchange reach, the daily call on the river, and the amount of water that is proposed to be exchanged. Using these data, an evaluation can be made of the timing, amount, and efficiency of a proposed exchange. This is one of the principal components of the water supply plan that has been extensively evaluated through the LWS point-flow and exchange model; discussion of the model will be covered in Part 3 of this blog series.
4.3 Existing River Diversion and Off-Stream Storage
The principal objective of this water supply plan is to capture available water at an upstream location and also to move available water to the same upstream location to help maximize beneficial use by using two distinct locations for diversions from the river. Because water is being diverted at two distinct locations, providing sufficient storage at each location is paramount to the success of this project. Storage at Iliff Reservoir is proposed to allow exchanges upstream and, in addition, there is an existing stream diversion on the South Platte River (“Prewitt Inlet Canal”), as well as existing storage (“Prewitt Reservoir”), to facilitate both exchanging water and diverting native water from the South Platte River and then storing the water (Figure 6). The Prewitt Inlet Canal has a decreed diversion right of 695 cfs, while the Prewitt Reservoir has a decreed capacity of 32,300 ac-ft. These structures currently serve a number of irrigation district in the Lower South Platte River basin, generally providing supplemental irrigation water later in the season as South Platte River flows decline and become insufficient to meet late-season irrigation needs. For this project, the diversion structure, delivery canal, and existing storage have to provide additional benefits for the project as there is not enough either direct flow or storage capacity to fully develop the proposed water supply project, as the other decreed uses and users in both structures have to be satisfied prior to the use of these facilities for the proposed new water supply project.
Figure 6: Prewitt Canal & Reservoir
Figure 7: Water Delivery System to Fremont Butte Reservoir.
In evaluating the water supply availability, it has been determined that the river diversion is adequate for all historic and proposed new uses, as well as exchanged water from Iliff Reservoir. In addition, since Prewitt Reservoir is used to provide irrigation water, sufficient storage space is not regularly available to meet the storage needs of this proposed water supply project. However, a portion of Prewitt Reservoir storage will need to be used for temporary forebay storage until it can be delivered to a new reservoir south of Prewitt (Figure 7).
Because of these factors, the project needs to have additional new storage in combination with some use of Prewitt Reservoir for forebay storage and expansion of the diversion capability of the Prewitt Inlet Canal to continue to provide its historic uses plus the proposed new uses.
4.4 New Off-Stream Storage
To fully utilize the Prewitt Inlet Canal and Reservoir for all uses, additional direct flow capacity and storage had to be evaluated using the LWS point-flow and exchange model, which will be described in the final part of this 3-part blog series. This analysis led to the need to have new, permanent storage that can be dedicated to the proposed new water supply project. Since only limited storage may be available in Prewitt Reservoir at times, it will only be used for forebay storage prior to being pumped and piped to a new reservoir at Fremont Butte. This reservoir will have a storage capacity of approximately 72,000 ac-ft and is located approximately 10 miles south of Prewitt Reservoir (Figure 6, above). Fremont Butte Reservoir will be the terminal storage facility on the downstream end of the water delivery pipeline to Parker and to LSPWCD upstream agricultural users.
The new Fremont Butte Reservoir is being designed at the primary water storage facility that can deliver water to: (a) Parker through a water delivery line to Rueter-Hess Reservoir; (b) LSPWCD users upstream of the reservoir through deliveries in the upstream end of the PWSD water delivery line; and/or (c) LSPWCD users downstream of Prewitt Reservoir by using the Fremont Butte pumping and piping system as a two-way line (Figure 7).
4.5 Water Delivery and Treatment System
The PWSD water delivery pipeline will extend from Prewitt Reservoir to Parker’s Rueter-Hess Reservoir as terminal storage to manage the ultimate municipal use in PWSD’s water supply system. However, LSPWCD may also use the upstream portion of the pipeline, by upsizing the pipeline diameter, to deliver water either back to the South Platte River upstream of the Prewitt Inlet Canal or to recharge structures in the South Platte River alluvium to provide delayed accretions to river flows that can then subsequently be used to augment out-of-priority depletions from alluvial well use for irrigation. LSPWCD may also use the Prewitt/Fremont Butte pipeline to deliver water back to the South Platte Rivers for its users downstream of the Prewitt Inlet Canal. These uses of the PWSD pipeline are shown in Figure 7.
Through the Iliff Reservoir, which can serve LSPWCD users downstream of that area, and the proposed water delivery mechanisms from Fremont Butte Reservoir, LSPWCD can provide agricultural irrigation water to its users from approximately Wiggins to the Nebraska state line, a distance of approximately 115 river miles.
Because of the water quality issues with Lower South Platte River water associated with elevated TDS concentrations and other mineralization, and the strict water quality controls on water sources that can be stored in Rueter-Hess Reservoir, a reverse osmosis (“RO”) plant needs to be built to reduce TDS concentrations to acceptable, potable concentrations before the delivery of this water into Rueter-Hess Reservoir. It is proposed that the RO plant be built on Colorado’s eastern plains as land costs are less than in the Denver metropolitan area and it will be easier to dispose of the RO brine, most likely through either evaporation ponds or deep well injection.
Not all of the water will be treated by RO, rather a split-stream process will be used whereby approximately half the water will be treated, while the other half will bypass the plant and be mixed again downstream of the plant. By doing so, treatment costs are minimized and the water will have its TDS concentration reduced to an acceptable level, yet the water will still be raw when it is discharged to Rueter-Hess Reservoir. The actual split-stream mix can be varied with time, as the TDS concentration in the South Platte River varies considerably, with very low concentration that require little to no treatment during spring runoff (typically less than 200 milligrams per liter (mg/L), while the TDS concentrations can approach, or be greater than, 1,000 mg/L during low-flow periods.
The final part of our three-part blog will describe the workings of the point-flow/exchange model created by Lytle Water Solutions (LWS).
If you have questions or would like additional information on creative solutions to water issues, please give us a call (303-350-4090) or send us an email at lws@lytlewater.com.
Bruce Lytle, P.E. bruce@lytlewater.com
Chris Fehn, P.E., P.G. chris@lytlewater.com
