Drivers of agricultural development and salinity in California's Salinas Valley watershed: A multi-decadal analysis

Coevolution of water demand and supply in the Salinas Valley beginning of the 20^th century is about a pathway to irrigated agriculture triggered by vegetable crop production and adoption of turbine pumps enabling extensive groundwater extraction across the Valley. We examine evolution of the Valley agricultural water use through a historical lens from 1929 to the present connecting feedbacks between the water demand, water supply and biophysical environment systems. We use mediation analysis to address the question of how and why the water management systems are related over time. Using a dynamic factor analysis, we ensembled a weighted multi-crop index to simulate total irrigated area based on cropping patterns and crop values. We found that favorable trade conditions for vegetables, wine grapes, berries and hay drive water demand in the Valley. Increase in water demand in the early part of the 20^th century was largely met by groundwater pumping. However, declining groundwater levels resulted in seawater intrusion and reduced groundwater supplies. Thus far, the preferred policy in response to the reduced groundwater supply has been increased investments in regional water supply infrastructure and farm-level water conservation technologies. Water demand has decreased largely due to the adoption of water conservation technologies; however, a rebound effect is that irrigated crop acreage has also increased. Overall, the coevolution analysis of agricultural developments indicated that a) land use changed over time such that seasonal vegetables, berries, nursery crops, wine grapes and orchard areas have increased, b) water demand was managed through implementations of water conservation technology and investments in alternative water supply infrastructure, c) technology mediated outcomes that reduced total dissolved solids (TDS) loading to surface water through implementation of drip, surge-flood irrigation and tailwater return systems, d) significant threats to nitrate (NO₃-N) in groundwater are associated with seasonal vegetables, berries and irrigated pasture and drip irrigation, e) water, nutrient and salt management policies had benefits for river TDS concentrations while they had no effect on groundwater NO₃-N concentrations and only short-term effects on groundwater elevations.