Sand Hills and Dissected Plains Water Budgets

Science Center Objects

A detailed annual water budget is invaluable for effective interrelated water management, particularly in river valleys where streamflow and crop demands are key components of the budget. Extraction of groundwater or diversion of surface water can affect flow to streams, wetlands, and other surface-water bodies. We are providing new insights into the water budget by estimating quantities of groundwater recharge and stream base flow and relating them to soil type, land use, and geology on a local scale for two small drainages, one in the Sand Hills and one in the Dissected Plains.

We are determining water budgets for two distinct regions in Nebraska: the Sand Hills and the Dissected Plains. Although these locations are within other regional-scale studies (Elkhorn-Loup Model), and recharge has been measured at different sites, no study has analyzed the integrated water budget on a local scale. We are estimating quantities of groundwater recharge and stream base flow and relating them to soil type, land use, and geology at these local scales to provide new insights into local water budgets.

A detailed annual water budget is invaluable for effective interrelated water management, particularly in river valleys where streamflow and crop demands are key components of the budget. Extraction of groundwater or diversion of surface water can affect flow to streams, wetlands, and other surface-water bodies. This impact can be assessed and mitigated if all of the components of the water budget are quantified. However, two of the largest components of the groundwater budget, groundwater recharge and base flow to streams, are spatially variable, and are quantified using varying techniques. 

Topographic regions within Nebraska
Topographic regions of Nebraska.

We are constructing an annual water budget for two Elkhorn-Loup Model (ELM) area drainage basins, including quantitative analysis of the most important components of the water budget: groundwater recharge and stream base flow. 

We focused on two distinct regions: 

  • —Sand Hills: Upper reach of Holt Creek in Holt County
  • —Dissected plains: South Loup River near Arnold

Five piezometers and one lysimeter were installed in each region. For streamflow data, a temporary streamgage was installed at Holt Creek. In the Dissected Plains region, the USGS streamgage at Arnold was used. Data collected at these sites is being used to determine recharge, evapotranspiration, and baseflow. Data also will be used as calibration and observation points for each specific model.

Do small streams in the Nebraska Sand Hills and Dissected Plains receive their base flow from local recharge, from regional groundwater flow, or from a combination of both?

Sand Hills region Nebraska water budget study area
Sand Hills region study area.

To answer this question, we have constructed water budgets that include several components:

  • Inputs: precipitation, imported water for irrigation, groundwater inflow, and percolation from surface-water bodies.
  • Outputs: runoff, plant consumption, evaporation, base flow to streams, and groundwater outflow.

Several components of the water budget (local precipitation, streamflow, irrigation pumping and surface-water deliveries, and vegetation demands) can be measured or calculated from existing data or data collected during the study. The amount and spatial distribution of groundwater recharge and base flow, however, is more difficult to assess.  

Ground-Water Recharge Estimates

We are estimating groundwater recharge using

  • Passive drainage lysimetry,
  • chloride mass balance, and
  • water-table fluctuations.
Dissected Plains region Nebraska water budget study area
Dissected Plains region study area.

Using these methods together allows us to overcome common problems of spatial and temporal representativeness that often plague traditional water-balance investigations.

Base Flow

Streamflow is measured at the gaging station within each region. The daily flow time series at each gage will be analyzed using a hydrograph base-flow separation method.

Piezometers

Piezometers were installed at various locations within each region. Data from the piezometers is being used to map the water table and analyze the flow system. 

Numerical Models

Recharge measurements are being used to calibrate a recharge analysis using the Soil Water Balance (SWB) code for the region. The SWB code uses land use, soil properties, and flow direction with daily weather data to track the soil-water budget and estimate the deep percolation that may become groundwater recharge. Components of the soil-water budget include precipitation infiltration, inflow from other cells, snow melt, potential and actual plant evapotranspiration (ET), and change in soil storage. Temperature and precipitation data collected at each site or downloaded from an existing weather station will provide climate data input for SWB. Point readings made using the chloride mass balance, lysimetry, and water-table fluctuation methods will be used to calibrate the SWB model and better estimate recharge in areas with no direct measurements. The SWB output includes recharge and evapotranspiration grids that will be used as recharge arrays in the groundwater model.

The SWB grids will be used with streamflow and base-flow estimates and piezometer readings to build a local, high-resolution MODFLOW simulation of groundwater flow for each site area. This model will include streams, recharge, lateral flow from the regional flow system, and any irrigation or domestic wells in the area. As much as possible, the model will use parameters from previous regional models such as the Elkhorn and Loup Model Phase I and Phase II.