1. Gerrit Schoups*,†,
2. Jan W. Hopmans*,‡,
3. Chuck A. Young*,
4. Jasper A. Vrugt§,
5. Wesley W. Wallender*,
6. Ken K. Tanji*, and
7. Sorab Panday¶

+Author Affiliations

1. ^*Hydrologic Sciences, Department of Land, Air, and Water Resources, University of California, Davis, CA 95616; ^§Earth and Environmental Sciences Division, Los Alamos National Laboratory, Los Alamos, NM 87545; and ^¶Hydrogeologic Inc., Herndon, VA 20170

1. Communicated by William A. Jury, University of California, Riverside, CA, September 6, 2005 (received for review April 29, 2005)

Abstract

The sustainability of irrigated agriculture in many arid and semiarid areas of the world is at risk because of a combination of several interrelated factors, including lack of fresh water, lack of drainage, the presence of high water tables, and salinization of soil and groundwater resources. Nowhere in the United States are these issues more apparent than in the San Joaquin Valley of California. A solid understanding of salinization processes at regional spatial and decadal time scales is required to evaluate the sustainability of irrigated agriculture. A hydro-salinity model was developed to integrate subsurface hydrology with reactive salt transport for a 1,400-km² study area in the San Joaquin Valley. The model was used to reconstruct historical changes in salt storage by irrigated agriculture over the past 60 years. We show that patterns in soil and groundwater salinity were caused by spatial variations in soil hydrology, the change from local groundwater to snowmelt water as the main irrigation water supply, and by occasional droughts. Gypsum dissolution was a critical component of the regional salt balance. Although results show that the total salt input and output were about equal for the past 20 years, the model also predicts salinization of the deeper aquifers, thereby questioning the sustainability of irrigated agriculture.

• regional hydrology
• salinization
• vadose zone

Salinization affects ≈20–30 million hectares (ha) of the world's current 260 million ha of irrigated land (1, 2) and limits world food production (3). Salinity reduces water availability to plants (4) by the accumulation of dissolved mineral salts in waters and soils due to evaporation, transpiration, and mineral dissolution. Subsequent salt leaching leads to salt buildup in both shallow groundwater below the plant root-zone (RZ) and deeper groundwater bodies (aquifers). The San Joaquin Valley, which makes up the southern portion of California's Central Valley, is among the most productive farming areas in the United States. However, salt buildup in the soils and groundwater is threatening its productivity and sustainability.

Currently, there is a good understanding of the fundamental soil hydrological and chemical processes (5) that control soil and groundwater salinity. Much of this understanding was achieved by using modeling approaches that consider the hydrology and soil chemistry separately, that assume simplified steady-state flow for spatial scales not larger than the field, and that only consider the RZ. However, recent research (6–11) has shown that soils must be fully coupled with the vadose zone and groundwater systems for regional-scale studies, especially in areas where groundwater tables are shallow or groundwater pumping is used (12). Innovative predictive tools are needed that can be applied at the regional scale and at the long term, so that the sustainability of alternative management strategies can be evaluated. For this purpose, an integrated regional-scale hydrosalinity model was developed to fully couple the hydrology and salt chemistry of the vadose zone with the groundwater system. This model enables us to reconstruct historical changes in soil and groundwater salinization in general and for the western San Joaquin Valley in particular (13).