Estimating agricultural deep drainage lag times to groundwater: application to Antelope Valley, California, USA

Abstract

Estimates of groundwater volumes available in semiarid regions that rely on water balance calculations require the determination of both surface to groundwater lag times and volumes from irrigation or rainfall initiated recharge. Subsurface geologic material hydraulic properties (e.g. hydraulic conductivities, water retention functions) necessary for unsaturated flow modelling are rarely available as are the instrumented field tests that might determine such lag times. Here we develop a simple two‐parameter (specific yield, Sy, and pore‐size distribution index, λ), one‐dimensional unsaturated flow model from simplifications of the Richards equation (using the Brooks‐Corey relationships) to determine lag times from agricultural deep drainage associated with the irrigation of alfalfa hay and various row crops in the Antelope Valley of California, USA. Model‐predicted lag times to depths of 85 m bgs (below ground surface) were similar to that measured in a 2‐year ponded recharge field trial, slightly overestimating that measured by approximately 15% (0.51 vs 0.44 years). Lag time estimates were most sensitive to estimated deep percolation rates and roughly equally sensitive to the model hydraulic parameters. Generally, as subsurface material textures coarsen towards larger Sy and λ values for all Sy >10%, lag times progressively increase; however, at Sy <10%, lag times decrease substantially suggesting that particular combinations of Sy and λ values that may be associated with similarly textured materials can result in the prediction of different lag times for Sy approximately 10%. Overall, lag times of 1–3 years to a depth of 69 m bgs were estimated from deep drainage of agricultural irrigation across a variety of irrigation schedules and subsurface materials. Copyright © 2012 John Wiley & Sons, Ltd.

Vadose zone lag time and potential 21st century climate change effects on spatially distributed groundwater recharge in the semi-arid Nebraska Sand Hills

“…The unit hydraulic gradient assumption is also valid when considering the relatively small changes in GR p associated with the decadal time step used. However, selection of this large time step does have the potential to inflate estimates of the lag times of the draining water's center of mass, since wet periods (with duration of days to weeks) could significantly increase GR p rates and vertical velocities (see Grismer, 2013). Thus, we consider the lag time estimates achieved with this approach as conservative (large), and recommend that it be used as a first-order approximation with a decadal (or longer) time step.…”

“…The saturated hydraulic conductivity, the most important of the various hydraulic parameters (Stephens, 1995), can vary by orders of magnitude depending on geologic heterogeneity. Grismer (2013) states, "determination of GR rates and lag times to groundwater supplies at depth remains an iterative process that includes model and estimate refinement as additional information is developed." In fact, a very small number of regional-scale hydrological modeling studies that include the vadose zone have attempted to match soil pressure heads or water contents during calibration, and constraints from these measurements do not result in uniqueness of parameter values (Brunner et al, 2012).…”

Impacts of thickening unsaturated zone on groundwater recharge in the North China Plain

“…(e.g., Grismer, 2013;Rossman et al, 2014), are key factors affecting temporal variations in recharge. However, most previous studies equate drainage or percolation below the root zone and recharge at the water table and commonly ignore time lags between the two (e.g., Kendy et al, 2003;Lu et al, 2011).…”

Rainfall leaching is critical for long-term use of recycled water in the Salinas Valley

“…Water balances have been used to estimate deep percolation rates from desert alfalfa hay production (Bali et al 2001;Grismer 2012) and from Sonoma County wine grape production (Grismer and Asato 2012) and also to corroborate field-measured soil profile drainage rates for avocado and citrus orchards on the central California coast (Grismer et al 2000).…”