Modeling groundwater table and runoff in self-organizing hydrologically sensitive areas

Understanding the hydrology of hydrologically sensitive areas (or runoff source areas) is crucial for evaluating and predicting runoff and the environmental fate of applied chemicals. However, while modeling these areas, one must deal with an overwhelmingly complex, coupled nonlinear system with feedbacks that operate at multiple spatiotemporal scales. Sufficient detailed information on the physical environment that these models represent is often not available. Consequently, the simulation's results, even after extensive calibration, are often disappointing. Fortunately, self-organization of hydrological systems' makes it possible to simplify watershed models and consider the landscape functions instead of small-scale physics. These simplified (or surrogate) models provide the same or better objective results than their complex counterparts, are much less data-intensive, and can be used for engineering applications and planning purposes.

This study aims to experimentally expose the landscape hydrological self-organization of a periodically saturated variable source area with a shallow perched water table and a humid climate. The study site is a four-hectare runoff source area near Cornell University, Ithaca, NY, US. The saturated hydraulic conductivity is greater than the rainfall intensity. The area has a single outlet through a notched weir, and the only inflow is from precipitation. We analyzed observed water table heights and field outflow and found the theory behind the self-organization of runoff processes specific to that landscape type. We determined a priori the thresholds for runoff in a surrogate model using the soil moisture retention curve. 

Weir measurements showed that outflow on the day following rainfall had decreased by orders of magnitude, indicating the soil water had returned to static equilibrium. Under the equilibrated state, established theory indicates that the matric potential decreases linearly with depth above the shallow groundwater. The matric potential (and thus the retention curve) determined the soil water distribution. Another property from the whole field perspective is that excess rainfall above saturation becomes runoff.

The reason for self-organization of the source area was that the soil moisture retention curve (which is similar for the whole source area) determined daily both the soil moisture content and the water table change using rainfall and evaporation as drivers. Since the source area behaved similarly, a simple surrogate water balance could predict the aggregated area's hydrological behavior. The nonlinear and small-scale physics associated with the field's complexity determined the rate that equilibrium is reached, which is always less than one day due to high macropore conductivity, greatly simplifying surrogate models that make daily predictions.