We examine potential regional-scale impacts of global climate change on sustainability of irrigated agriculture, focusing on the western San Joaquin Valley in California. We consider potential changes in irrigation water demand and supply, and quantify impacts on cropping patterns, groundwater pumping, groundwater levels, soil salinity, and crop yields. Our analysis is based on archived output from General Circulation Model (GCM) climate projections through 2100, which are downscaled here to the scale of the study area (~30 km across). We account for uncertainty in GCM climate projections by considering output from two different GCM’s, each using three greenhouse gas emission scenarios. Significant uncertainty in projected precipitation translates into uncertainty of future water supply, ranging from an increase of 10% to a decrease of 30% in 2100. On the other hand, temperature projections are much less variable, resulting in consistent projections of crop water demand for all climate change scenarios. Crop water demand is expected to change very little, due to compensating effects of rising temperature on evaporative demand and crop growth rate. Reductions in surface water supply are projected to be offset by groundwater pumping and land fallowing. Simulations of subsurface flow and salt transport with a regional-scale hydro-salinity model suggest a small expansion in salt-affected area, compared to current conditions. However, in all scenarios salinity is expected to increase in downslope areas, thereby limiting crop production. This is especially significant given an anticipated demand-driven switch to high-value, salt-sensitive crops. Results show that technological adaptation, such as improvements in irrigation efficiency, may partly mitigate these effects.

The study of water migration and contaminant transport in soils is of fundamental importance in hydrologic science. Movement of agrochemicals (nitrates, atrazine) and industrial solvents (TeE, carbontetrachloride) to the groundwater is of great public concern. In order for these chemicals to reach the groundwater, they must pass through the vadose zone. Thus, in order to predict travel times of contaminants toward the groundwater or to understand the underlying transport process, we need to characterize and sample the unsaturated zone. The need for an economical, readily available monitoring system with a broad range of applications is required by the practicing groundwater scientist and vadose zone hydrologist, given the growing recognition of the interdependence of the unsaturated zone and saturated zone processes. At present, there are few, if any, cost effective and practical field monitoring devices which incorporate the sophisticated sampling and data acquisition array necessary for representative monitoring of the vadose zone.

As concern for a save environment and groundwater quality increases, the importance of an accurate soil hydraulic description of the combined unsaturated-saturated porous system is increasingly recognized in the fields of environmental engineering and groundwater hydrology. With this wider interest, the spatial scale of interest has shifted to dimensions as large as a watershed, and to depths from the rooting zone to the groundwater. This trend in increasing larger spatial scales of the vadose zone brings along with it the presence of increasing soil heterogeneity within the considered system. Therefore, methodologies need to be developed that allow for a rapid and accurate characterization for the soil hydraulic properties and its spatial variability.

The objectives of the WRC-sponsered research was to develop a single sampling unit which can be used to sample soil solution, but at the same time monitors continuously the soil water potential. The second objective was to demonstrate the potential application of this combined tensiometer-soil soluton extraction probe to estimate in situ the soil water retention and unsaturated hydraulic conductivity functions.