LNWB Ch06 Soil Processes and Inputs

Overview:
Water, in its many forms is one of Whatcom County’s signature features from snow-capped mountains,to our rainy climate, salmon-bearing streams, wetlands, lakes, marine waters, and marine shorelines. Five distinct hydrologic components control the storage and movement of water through the canopy and soils: canopy interception store (green trees), upper soil zone (vadose zone, brown soil fill) store, groundwater saturated zone (gray soil fill), channel flow (blue), and artificial drainage (blue line from agriculture to channel). Surface water inputs from direct precipitation, throughfall through the vegetation canopy, and irrigation are taken as input to the unsaturated, or vadose zone soil store. The unsaturated portion of the upper soil layer (brown), or vadose zone, is shown with recharge water (blue downward line) infiltrating the surface layer of soils, draining through the unsaturated zone (brown), to recharge the saturated zone (gray). The thickness of the vadose zone changes as the water table level (hashed gray and brown interface) shifts up and down, depending on the water held in the saturated zone. Based on the input and storage in the vadose zone, recharge to groundwater (gray, saturated zone) and surface water runoff is calculated. The vadose zone soil store is decreased by artificial drainage, representing ditch and tile drains that remove water directly from the vadose zone soil store to channels. The vadose zone soil store calculation also accounts for potential upwelling from groundwater where the water table is shallow. The groundwater saturated zone calculations account for recharge, upwelling and groundwater pumping and produce baseflow as an output. In the Lower Nooksack Water Budget, baseflow is defined as the outflow from the saturated zone and referred to as groundwater contribution; and baseflow and surface runoff are combined to calculate channel flow.

Purpose:
The baseflow in streams is supported by the gradual drainage of groundwater in shallow aquifer systems. The rate of this drainage depends on the amount of water stored in shallow aquifers (depth to water table) and the hydraulic properties of the aquifer, specifically the lateral hydraulic conductivity, or its depth integral, transmissivity. The amount of water stored depends on recharge, the vertical movement of water through unsaturated soils from the surface into the shallow groundwater. The rate of recharge is determined by the supply of water above. This is a function of whether surface water input is retained in the soil zone where it is taken up by plant roots and becomes evapotranspiration, or whether it infiltrates beyond the root zone and percolates to aquifers. These processes depend on the properties of the soils, such as porosity, field capacity, and hydraulic conductivity. The representation of the hydrologic processes of recharge and drainage to baseflow on a drainage scale is done using estimates based on measured data at point locations, as well as soil texture information. As more data is collected, information about subsurface processes can be incorporated into the model representation.

For the Lower Nooksack Water Budget soils parameters, soils data was compiled from both local and federal datasets. Using data available from the Natural Resource Conversation Service (NRCS – formerly the Soil Conservation Service) soils databases (NRCS; SSURGO and STATSGO (www.soilsdatamart.gov)), we have used estimates of averaged soils parameter values over each drainage area as data inputs for the hydrology model compiled in previous work (Tarboton, 2007). These soil parameters include plant available soil moisture, soil depth, hydraulic conductivity, and wetting front suction. Earlier calibrations of Topnet-WM showed that the most sensitive and therefore important soil parameters controlling baseflow movement are saturated soil store sensitivity (f) and soil profile lateral conductivity or transmissivity (To). The Lower Nooksack transmissivity parameters were derived from aquifer hydraulic conductivity values for specific wells, completed within shallow near surface aquifers, as described in the U.S. Geological Survey (USGS) Lynden-Everson-Nooksack-Sumas (LENS) Study (Cox and Kahle, 1999). Although the variability in well data is high given the heterogeneity of glacial and alluvial deposits, interpolating available well data to derive drainage average values captures the drainage level heterogeneity. Here changes in average depth to water table described in the Department of Ecology Study, Nooksack Watershed Surficial Aquifer Characterization (Tooley and Erickson, 1996), were used. Water movement through the surficial aquifer is assumed to decrease exponentially as the depth to the water table increases based on the Topmodel algorithm (Beven, et al., 1995a).

This resource is a subset of the Lower Nooksack Water Budget (LNWB) Collection Resource.