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

<p>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.</p><p>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. </p><p>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.</p><p>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.</p>

Identification of Hydrologically Sensitive Areas Considering Watershed Process Dynamics

Hydrologically sensitive areas (HSAs) largely control watershed response to rainfall, along with water and pollutant transport processes. Thus, their identification is critical in watershed management planning. Although watershed processes have been studied enough to provide a good understanding of HSA dynamics, only a few concepts and methods are available for HSA delineation, and they rely on spatial indices that do not consider temporal variation in hydrologic processes. This study introduces alternative concepts and methods to delineate HSAs. Three unique maps showing watershed dynamics were created using the outputs of a long-term hydrologic simulation implemented with a grid-based distributed model. The spatial distributions of HSAs identified using the newly proposed methods were compared with those of topographic indices. Results demonstrated that the new methods highlight transport processes, such as routing (or connection) and travel time, and the roles of soil and land covers, which have not been the focus of other concepts and approaches for HSA identification. In contrast to topographic index-based approaches, the proposed methods provided HSA boundaries with clear physical meanings to improve the interpretability and applicability of HSA maps. The methods are expected to enhance our ability to tackle water issues for improved water resource management by providing unique concepts and alternative ways to explicitly delineate HSAs. Keywords: Grid-based distributed model, Hydrologic connectivity, Hydrologically sensitive area, HYSTAR, Time-area method, Topographic wetness index.