WHY GROUNDWATER MATTERS: AN INNOVATIVE LOOK AT PUMPING, IRRIGATION, SYSTEM DYNAMICS AND SUSTAINABILITY

Regional impacts from irrigation and groundwater pumping have been the subject of much research. Connections between pumping, drawdown, increased evapotranspiration and decreased runoff are well established; however, the effect of groundwater fed irrigation on system dynamics has not been explored thoroughly and has potentially important implications for the ability of a watershed to respond to stress. We use a fully-integrated hydrological model, ParFlow, to simulate moisture-dependent irrigation in the Little Washita basin USA. Three, twenty-year simulations are completed for the ~1,700 km² domain using hourly historical meteorological forcings from 1990 to 2009 for three scenarios with varying farm coverage. As would be expected, analysis shows spatiotemporal variability at a range of frequencies and correlation lengths. However, local behavior is highly variable and can be impacted by a number of physical parameters and transient forcings. To quantify system behavior, a classification scheme was developed that groups time series based on annual trends and the ratio of intra-annual variability to inter-annual variability. This scheme is demonstrated using time series of end of month water table depth. Analysis, of the baseline scenario with no agriculture shows spatial organization of classification groups. Along the river, where groundwater is shallow, water table depths have a strong intra-annual cycle but relatively constant long term average. Conversely, near the hilltops, where the water table is deepest, monthly variability is small compared to the longer term oscillations. Contrasting classifications from the baseline case to the irrigation scenarios, there are significant shifts in water table dynamics. The imposition of an annual pumping cycle in a small portion of the domain (~10 – 23%) leads to larger intra-annual variability throughout much of the study area and expands areas where intra-annual variability is larger than inter-annual variability. Results show that connections between groundwater depth and irrigation demand lead to feedbacks between pumping, and demand. Thus, groundwater pumping exercised in a small portion of the watershed influences system dynamics far beyond the well and ultimately changes the nature of sustainable agriculture throughout the domain.