| Title |
Investigators | Department | Objectives | Approach Keywords | Progress Reports | Impact Statements | Publications | |
Project * COL00725 | |
| Title | The Critical Groundwater Link between Irrigated Agricultural Water Use and Fish Habitat on the High Plains |
| Investigator(s) | Durnford, DS; |
| Department | Civil and Environmental Engineering |
| Objectives | The development of intensive, irrigated agriculture, particularly large capacity center-pivot irrigation, has precipitated ecological changes that have had a profound impact on the ecohydrology of the West, including changes in riparian and in-stream habitat for fish and wildlife. Many small rivers on the high plains are gaining streams, i .e. the baseflow in these rivers is sustained by groundwater. Intensive pumping by large capacity irrigation wells from alluvial aquifers reduces the recharge to streams and rivers and may, at times, reverse the hydraulic gradient so that stream depletion occurs, i.e. the stream becomes a losing stream. This can negatively impact floodplain wetlands and backwater sloughs that serve as aquatic and riparian habitat for fish and wildlife, as well as change in-stream hydrology that could lead to extirpation of critical, threatened or endangered species. This project will study the impact of agricultural center-pivot pumping from the High Plains aquifer on the viability and sustainability of a threatened fish population in the Arikaree River. The study site presents an ideal opportunity to study the effects of irrigated agriculture on an important ecosystem. There is a clear hydrologic link between the water pumped for irrigation and the fish habitat in the Arikaree River. The river is sustained by groundwater. The drawdowns in the aquifer, and consequent stream depletions, modify the stream hydrology, drying up sections of the stream habitat, reducing backwater sloughs and stream depths needed for fish spawning and rearing. This project will model the critical hydrologic linkages between center-pivot pumping for irrigation from an alluvial aquifer and stream habitat required for survival and reproduction of a threatened fish species. The overall goal is to conserve the habitat (defined by spatial and temporal water quality and quantity variables) of a threatened plains fish while simultaneously satisfying the needs of irrigated agriculture, without negatively impacting the local agricultural economy or the water rights of the agricultural water users. |
| Approach | The study site is the Arikaree River Basin in Yuma County, CO., which is representative of many agricultural areas that extract water from the High Plains Aquifer. In the Northern High Plains Aquifer (which includes the study site), an estimated 5.3 million acre-feet of water have been mined in the last five years (1998-2003), resulting in a decline of nearly six feet in water levels. It is hypothesized that sustained drought conditions in Colorado, coupled with the current level of groundwater pumping from the High Plains Aquifer, could result in the extirpation of state-listed threatened or endangered plains fish species such as the brassy minnow on the Eastern Plains. Thus, there is a clear potential existing and future conflict between a rural, agriculturally based economy and the future of a threatened plains fish population. The proposed research will: (1) establish a groundwater well/stream stage monitoring network to determine the hydrologic linkages between irrigation pumping, the groundwater response, stream levels, and habitat characteristics in the Arikaree River, (2) develop and calibrate a numerical groundwater/surface water model, (3) use knowledge of the hydrologic linkages and habitat criteria to predict the extirpation of threatened and endangered plains fishes, under various what if scenarios, (4) identify options that meet the water needs of the fish population without damaging the profitability of irrigated agriculture, and (5) cooperate with farmers and water managers to develop plans to ameliorate potential deleterious effects of agricultural water use on fish habitat and survival of the species-of-interest, while preserving the regional agricultural economy. |
| Keywords | hydrology, groundwater, fish habitat, stream depletion, stream ecology, ecohydrology, conjunctive water use, irrigated agriculture, stream/aquifer interactions |
| Progress Reports | |
| 2004 | The project site is the Arikaree River basin in Eastern Colorado. Flow in the Arikaree River is entirely sustained by the Ogallala Aquifer and it is also the last stronghold in Colorado of the brassy minnow, a state-listed threatened fish species. Since water levels in the aquifer are declining, it is expected that continued high capacity pumping from the aquifer will dry critical reaches of the Arikaree River, negatively impacting fish survival. To address this issue, two questions must be answered initially. First, what is the extraction by the riparian phreatophyte vegetation and the impact of this extraction on the streamflow in the Arikaree River? Second, how much water is used by the center pivots near the river for irrigation and what is the impact of the center pivot irrigation systems on the baseflow of the river? To address these initial questions and provide input needed for later modeling efforts, a network of monitoring wells was installed this year. In addition to monitoring water levels in the aquifer, water levels in the stream and streamflows are being measured at preselected time intervals. This information will allow us to estimate inputs to a numerical groundwater model that will be developed in the second phase of this project and also provide water level data for model calibration. The ultimate goal of the project is to determine hydrologic linkages between the center pivots and the stream so that center pivots can be retired or irrigation temporarily curtailed to maintain minimum critical fish habitat during drought years. |
| 2005 | The study site for this project is the Arikaree River on the high plains of eastern Colorado. This is a small stream that is almost entirely sustained by the regional High Plains Aquifer and one that is habitat for a state-threatened fish, the brassy minnow. As part of an interdisciplinary project, we are investigating the effect of center-pivot pumping for crop irrigation on the stream hydrology and the stream ecosystem. We have completed an initial numerical groundwater model for the upper reaches of the study area near The Nature Conservancy Fox Ranch. The model was calibrated for steady state conditions using predevelopment water table data. We also completed a field study to predict the evapotranspiration rates of the phreatophytes near the river. From our preliminary field study and computer modeling, it appears that evapotranspiration of the riparian vegetation (primarily phreatophytes such as willows and cottonwoods) from the shallow alluvial aquifer near the stream is playing a much bigger role in the stream drying inter-season cycle than the large-scale center pivots pumping from the regional aquifer. On the basis of this preliminary modeling, we hypothesize that the high capacity center pivot irrigation systems are mining the regional High Plains Aquifer, resulting in the measured water table decline of about a foot per year. However, they do not draw down water levels in the stream significantly within a single growing season. This has important implications with regard to strategies based on irrigation forbearance to protect aquatic ecosystems from short-term drying during a summer season. We will model the center pivot irrigation effects with a large-scale regional model of the High Plains Aquifer using a coarse grid spacing. Assuming, however, that the stream stage variation within a season more closely reflects the evapotranspiration pattern for the riparian vegetation, we will model the alluvial aquifer, flood plain, riparian vegetation and river will be modeled on a smaller grid. The connection between the regional and local scale models will be through a general head boundary condition for the alluvial aquifer, which will reflect the long-term trend in regional water table declines. |
| 2006 | The goal of this project is to model the critical hydrologic linkages between center-pivot pumping for irrigation from an alluvial aquifer and stream habitat required for survival and reproduction of a threatened fish species. Decline of the Ogallala Aquifer due to groundwater pumping for center-pivot irrigation of crops is one of the most important environmental and economic problems associated with agriculture on the Great Plains. The aquifer sustains both an important agricultural economy and most of the aquatic and riparian ecosystems in the region, and both are in trouble. This project will predict the effects of a five-year drought on stream hydrology and fish persistence under various future scenarios. To date, we have completed the development and calibration of a large-scale (regional) numerical groundwater/surface water model. However, it has become clear that the temporal and spatial scale relevant to the fish habitat is much smaller than the regional-scale model. Therefore, we are writing code for a smaller scale model that will incorporate just the alluvium and time scales that are daily or weekly. We will imbed this new model in the regional model. |
| Impact | |
| 2004 | Agricultural economies and aquatic ecosystems on the High Plains of Colorado are largely sustained by the Ogallala Aquifer. Farmers in these areas subsist on very low profit margins and their crops are largely dependent on groundwater for irrigation. Rivers in these areas are also dependent on groundwater baseflow. In the last few decades, large scale center pivot irrigation has led to regional declines in the water table, resulting in reduced baseflow to the rivers and increasingly dry stream reaches. This results in negative impacts on aquatic/riparian ecosystems. In particular, some threatened fishes are declining. An example is the brassy minnow. This species was once widely distributed in northeastern Colorado. Now, its distribution is restricted almost entirely to about 50 km of the Arikaree River near the Kansas border. To sustain both a precarious regional agricultural economy dependent on groundwater for irrigation, and an aquatic/riparian ecosystem likewise dependent on groundwater for existence, it is clear that we must develop the knowledge required to predict tradeoffs in use of this scarce resource. This project will determine the hydraulic linkages between selected center-pivots and these critical river reaches, and develop a protocol for selecting wells that will have the most impact on fish survival. This information will be useful to water managers and irrigators. Possible solutions include permanent removal of center pivot systems that are most critical or irrigation forbearance during drought years to protect critical stream reaches. |
| 2005 | Both the phreatophyte vegetation withdrawing water from the alluvial aquifer and the high capacity center-pivot irrigation systems pumping from the regional High Plains Aquifer affect the stream hydrology and, hence, the aquatic ecosystem. However, it appears that the timing of the two effects on the stream is significantly different. In earlier work, there was a strong correlation noted between the pumping pattern for irrigation and the drawdowns and recovery measured in the stream. However, from our modeling, it appears that this correlation is not direct and, in fact, the primary effect of pumping is the long-term decline of the regional water table, which over years and decades will lower the influx to the alluvial aquifer. It does not, however, relate to the seasonal stream stage fluctuations directly. This has significant implications for irrigation forbearance, which is being proposed at many locations in the arid West as a response to drought when aquatic ecosystems are vulnerable. These early results would suggest that turning off an irrigation well may not result in a timely response of the stream. We will continue to investigate the relative importance of near-stream vegetation and high capacity pumping on the dynamics of the stream. |
| 2006 | We believe that our study site, the Arikaree River, is at a "critical threshold", as are other groundwater-sustained streams in the high plains. A high concentration of center-pivot irrigation systems, a declining water table, a strong link between groundwater pumping and stream baseflow, the presence of a state-threatened fish, a precarious local agricultural economy, an exploding population willing to pay high prices for water rights and finally, state and local agencies and local farmers primed by a recent severe multiyear drought make this a timely and important study. The long-term consequences of failing to research and protect the future of this river include the real possibility that there may soon be no river. The research proposed is especially timely because the State of Colorado is expected to spend over $62 million dollars over the next 15 years to take 30,000 center-pivot irrigated acres out of production in the basin of our study site to meet a recent interstate water compact. This "irrigation forbearance" program will enroll producers based only on the distance between the well and the river. Our research has already helped The Nature Conservancy identify center-pivot systems that will not only meet compact requirements, but also are most important in protecting core aquatic habitat. |
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