Long-Term Conjunctive Water Management for Agriculture with Groundwater Salinity

Abstract In California and other (semi-)arid regions, chronic groundwater overdraft and aquifer salinization limit irrigated agriculture, threaten crop yield, and reduce flexibility in groundwater management. Higher aquifer salinity reduces groundwater’s ability to irrigate in drier years, when fresher water is less available to dilute more saline groundwater. This paper employs a novel, nested hydro-economic optimization framework—a decadal stochastic quadratic optimization model within a 10-stage dynamic program—to analyze century-scale water and crop management for a salinizing aquifer with limited drainage and hydrologic variability, similar to conditions in California’s western San Joaquin Valley. Results shows that groundwater management with accumulating salinity differs fundamentally from freshwater aquifers, resulting in greatly reduced net extraction and earlier aquifer recharge to slow salinity accumulation and prolong the utility of groundwater. Key optimal strategies also include the strategic reduction of early perennial crop acreage to support salinity suppression. For an example basin, higher initial salinity of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" alttext="1,500 m g divided by normal upper L" display="inline" overflow="scroll"> <mml:mn>1,500</mml:mn> <mml:mtext> </mml:mtext> <mml:mi>mg</mml:mi> <mml:mo stretchy="false">/</mml:mo> <mml:mi mathvariant="normal">L</mml:mi> </mml:math> (versus <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" alttext="500 m g divided by normal upper L" display="inline" overflow="scroll"> <mml:mn>500</mml:mn> <mml:mtext> </mml:mtext> <mml:mi>mg</mml:mi> <mml:mo stretchy="false">/</mml:mo> <mml:mi mathvariant="normal">L</mml:mi> </mml:math> ) can greatly reduce long-term regional economic value. The economic rationale for recharge with deteriorating groundwater salinity transitions from a drought storage mechanism to a high-cost but still useful input for preserving perennial agriculture, with feasibility constrained by discount rates and climate. This work provides a strategic and quantitative approach for managing the irreversible degradation of groundwater resources.