Regional management of a stock pollutant: agricultural drainwater
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The paper addresses long-run drainage problems in San Joaquin valley irrigated agriculture. The region slopes downward from west to east. Several hundred feet below the land surface is the Corcoran clay layer which is relatively impervious to water and results in a perched, shallow water table aquifer. Irrigation water is typically applied in excess of crop ET resulting in water percolation below the rootzone. Some deep percolation is necessary to flush salts from the rootzone, however the bulk is generally due to nonuniform irrigation and infiltration. Deep percolation flows can cause the shallow water table to build up over time with consequent yield losses.Several management strategies are available: Deep percolation can be reduced by alternate irrigation systems, crop switching, moisture-stressing, and reduction of cropped area, and water can be withdrawn from the shallow water table aquifer and disposed in evaporation ponds. Producers have some incentive to adapt water conservation and drainage disposal measures when the water table begins to encroach their own land. Beyond this they have minimal incentive to reduce deep percolation flows in the absence of regulation since, with many users, the bulk of the damages will occur elsewhere. Regional drainwater management is considered here using a dynamic stock pollutant model with spatial variability. The analysis integrates an economic model of agricultural production with a hydrologic (finite-difference) model of groundwater flows. Both the value and the cost of reduced emissions are endogenous and can vary over both space and time. The first main issue addressed is long-run sustainability of the resource under common property usage: can a high level of agricultural production be maintained in the basin even in the absence of regulation? The results suggest yes provided growers have access to evaporation ponds or other disposal mechanisms. The second main issue relates to efficient management and specifically the extent to which upslope growers should mitigate emissions to reduce downslope damages. The results suggest both timing and spatial considerations. In particular, the results establish that drainage management should begin before explicit drainage problems occur. This delays – and may even – prevent drainage problems. Considerable spatial variation in efficient source control is also found even with what appears at first glance to be a minimal level of exogenous spatial variability in the underlying system. Conceptual reasoning as well as computational experiments suggest that this result arises as a combination of land slope, finite hydraulic conductivity, and discounting. This suggests a second critical dimension of managing agricultural drainwater beyond the timing dimension analyzed in the early stock pollutant studies. The implications of efficient management for regulatory policy are also considered. The results demonstrate how marginal user cost (future impacts) varies temporally and spatially, and demonstrate that efficiency implies a high degree of spatial variability in contrast to SJVDP proposed (uniform) regulatory standards. Efficiency-inducing policy instruments in both quantity form and price form are developed for both input-side regulation and output-side regulation.