Submitted to: Agricultural Water Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/17/2009
Publication Date: 5/1/2010
Citation: Shouse, P.J., Goldberg, S.R., Skaggs, T.H., Soppe, R.O., Ayars, J.E. 2010. Changes in spatial and temporal variability of SAR affected by shallow groundwater management of an irrigated field, California. Agricultural Water Management. 97(5):673-680. Interpretive Summary: In San Joaquin Valley, CA, the disposal of agricultural drainage waters poses significant environmental challenges. Efforts are underway to develop irrigation water management practices that reduce the volume of drainage generated. One such management strategy involves restricting flow in subsurface drains in an effort to raise the water table and induce the consumption of groundwater by crops. A potential complication with this management approach is that upward groundwater flow may move salts such as sodium into to root zone. Increasing the sodium content of soils often leads to reductions in soil permeability such that water flow and crop growth are restricted. In a three year field study investigating groundwater management, we found that sodium concentrations were indeed increasing. However, increases in other salts and the presence of certain minerals in the soil counteracted the normally harmful effects of increased sodium. We concluded that sodium hazard was not expected to become a limiting factor for long term shallow groundwater management and that the technology will be viable for future seasons. This research will assist regulators and growers seeking innovative water management practices that reduce agricultural drainage volumes in San Joaquin Valley, CA.
Technical Abstract: In the irrigated western U.S. disposal of drainage water has become a significant economic and environmental liability. Development of irrigation water management practices that reduce drainage water volumes is essential. One strategy combines restricted drainage outflow (by plugging the drains) with deficit irrigation to maximize shallow groundwater consumption by crops, thus reducing drainage that needs disposal. This approach is not without potential pitfalls; upward movement of groundwater in response to crop water uptake may increase salt and sodium concentrations in the root zone. The purposes for this study were: to observe changes in the spatial and temporal distributions of SAR (sodium adsorption ratio) and salt in a field managed to minimize drainage discharge; to determine if in situ drainage reduction strategy affects SAR distribution in the soil profile; and to identify soil or management factors that can help explain field wide variability. We measured SAR, soil salinity (EC1:1) and soil texture over 3 years in a 60-ha irrigated field on the west side of the San Joaquin Valley, California. At the time we started our measurements, the field was beginning to be managed according to a shallow groundwater/drainage reduction strategy. Soil salinity and SAR were found to be highly correlated in the field. The observed spatial and temporal variability in SAR was largely a product of soil textural variations within the field and their associated variations in apparent leaching fraction. During the 3-year study period, the percentage of the field in which the lower profile (90–180 cm) depth averaged SAR was above 10, increased from 20 to 40%. Since salinity was increasing concomitantly with SAR, and because the soil contained gypsum, sodium hazard was not expected to become a limiting factor for long term shallow groundwater management by drain control. It is anticipated that the technology will be viable for future seasons.