The Richards equation is unsuccessful at describing gravity‐driven unstable flow with nonmonotonic water content distribution. This shortcoming is resolved in the current study by introducing the moving‐boundary approach. Following this approach, the flow domain is divided into two subdomains with a sharp change in fluid saturation between them (moving boundary). The upper subdomain consists of water and air, whose relationship varies with space and time following the imposed boundary condition at the soil surface calculated by the Richards equation. The lower subdomain consists of an initially dry soil that remains constant. The location of the boundary between the two subdomains is part of the solution, rendering the problem nonlinear. The moving boundary solution was used after verification to demonstrate the effect of contact angle, soil characteristic curves and incoming flux on the dynamic water‐entry pressure of the soil, which depends on the soil's wettability, incoming flux at the soil surface and the wetting front's propagation rate. Lower soil wettability hinders spontaneous invasion of the dry pores and, together with a higher input flux, induces water accumulation behind the wetting front (saturation overshoot). The wetting front starts to propagate once the pressure building up behind it exceeds the dynamic water‐entry pressure. To conclude, the physically based novel moving‐boundary approach for solving stable and gravity‐driven unstable flow in soils was developed and verified. It supports the conjecture that saturation overshoot is a prerequisite for gravity‐driven fingering.

The moving-boundary approach, which has been successfully used to model stable and unstable 1-D flow in initially dry soils of various contact angles (Brindt & Wallach, 2017 https://doi.org/ 10.1002/2016WR019252), was extended here for 2-D flow. The wetting front is the plume perimeter that is partly formed by the capillary driving force, the remaining part by the combined capillary and gravity driving forces. The moving-boundary approach overcomes the limitation of the Richards equation for describing gravity-driven unstable flow with nonmonotonic water-content distribution. According to this approach, the 2-D flow domain is divided into two subdomains with a sharp change in fluid saturation between them-the wetting front (moving boundary). The 2-D Richards equation was solved for the subdomain behind the wetting front for a given flux boundary condition at the soil surface, while the location of the other boundary, for which a no-flux condition is imposed, was part of the solution. The moving-boundary solution was used after verification to demonstrate the synergistic effect of contact angle and incoming flux on flow stability and its associated plume shapes. The contact angle that hinders spontaneous invasion of the dry pores decreases the water-entry capillary pressure, ψ we , while the flux-dependent dynamic water-entry value, ψ wed , is even lower, both inducing water accumulation behind the wetting front (saturation overshoot). This innovative physically based model for the 2-D unsaturated flow problem for an initially dry soil of zero and nonzero contact angle using the moving-boundary approach fulfills several criteria raised by researchers to adequately describe gravity-driven unstable flow.

The recognition of treated wastewater (TWW) as an alternative water resource is expanding in areas with a shortage of freshwater (FW) resources. Today, most orchards in Israel are irrigated with TWW. While the benefits of using TWW for irrigation are apparent, evidence of its negative effects on soil, trees, and yield is accumulating. This study, performed in a commercial TWW-irrigated citrus orchard in central Israel, examined the effects of (1) soil-wettability decrease due to prolonged TWW irrigation on the spatial and temporal distribution of water content and associated chemical properties in the root zone; (2) the conversion of irrigation in half of the TWW-irrigated research plot to FW (2012) for soil reclamation. Electrical resistivity tomography surveys in the substantially water repellent soils revealed that water flow is occurring along preferential flow paths in both plots, leaving behind a considerably nonuniform water-content distribution. This was despite the gradual relief in soil water repellency measured in the FW plots. Four soilsampling campaigns (spring and fall, 2014-2016), performed in 0-20 and 20-40 cm layers of the research plot, revealed bimodal gravimetrically measured water-content distribution. The preferential flow led to uneven chemical-property distribution, with substantially high concentrations in the dry spots, and lower concentrations in the wet spots along the preferential flow paths. The average salt and nutrient concentrations, which were initially high in both plots, gradually dispersed with time, as concentrations in the FW plots decreased. Nevertheless, the efficiency of reclaiming TWW soil by FW irrigation appears low.

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<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>