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Task 2 — Recharge and Discharge
Regionally consistent estimates of mean annual recharge and discharge, including discharge by evapotranspiration (ET), pumpage, streamflow, and springflow will be developed. Preliminary estimates for discharge and recharge will be developed using available data; estimates will be updated, where possible, with new data acquired during this study. The mean annual estimates of recharge and discharge will be developed using the following:
Recharge
Spatially-distributed estimates of mean annual recharge will be developed using a two-step approach:
First, a basin-characterization model (BCM) will be applied for the entire regional area. The BCM is a relatively simple approach for regional analysis that uses a deterministic weekly or monthly water-balance method that includes the distribution of precipitation, estimates of potential ET, soil-water storage, and bedrock permeability.The BCM will be used with available geographic information system (GIS) data (digital elevation model, geology, soils, vegetation, precipitation, and air temperature). Second, a more detailed numerical rainfall-runoff model (INFILv3) will be used in selected basins identified as hydrologically significant in the BCM model. INFILv3 incorporates a daily water-balance model of the root zone. The daily water-balance includes precipitation (rain or snow), snow accumulation, sublimation, snowmelt, infiltration, ET, drainage, water-content change, runoff, surface water, and net infiltration. |
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Discharge
Spatially-distributed estimates of mean annual discharge will be developed by quantifying streamflow, springflow, pumpage and ground-water ET. Mean annual precipitation over discharge areas will be estimated and the source of streamflow into or out of major discharge areas will be determined. This data will be used to help partition phreatophyte ET rates into ground-water or surface-water components. Determining reasonably accurate estimates of mean annual discharge will include the following:
Springflow — Available springflow measurements will be compiled and field checked including characterization of channel and stream-source waters. Abiotic and biotic characteristics of approximately 60 springs will be quantified to develop a model relating discharge sources.
Pumpage — Current pumpage will be estimated using all available pumpage data from the State of Nevada Ground-Water Pumpage Inventories. For basins not inventoried, large irrigation use will be estimated from irrigated acreage delineated using remotely-sensed satellite data, estimated crop application rates, and any associated information from State well log and water-use permit databases.
ET in Ground-Water Discharge Area — Field ET measurements will be collected at a near-point scale (10 ft) using a dome technique to a plant community or fetch scale (500 ft) using Eddy-covariance or Bowen-ratio techniques. Additional Ground-Water/Surface-Water Partitioning — Water samples, soil cores, and plant-stem samples will be analyzed for stable isotopes (oxygen-18) to determine the likely contribution of ground water or surface water to ET. Samples will be collected seasonally from several representative areas where surface-water inflows are assumed to be a significant contribution to ET. |
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