How geomorphology shapes groundwater transit times at the hillslope scale?

<p>We investigate how geomorphological structures shape Transit Time Distributions (TTDs) in shallow aquifers. We show that the TTD is determined by integrated features of the groundwater structure and of the repartition of seepage in convergent/divergent hillslopes of constant slope. More specifically, the coefficient of variation of the TTD (standard deviation divided by the mean) scales linearly with the mean distance of the groundwater volume to the river. The extent and structure of seepage modify the groundwater contribution to the transit time distribution and increase its variability.</p><p>Extensive 3D simulations were performed to determine the TTDs synthetic convergent, straight and divergent hillslope models of constant slope. The recharge was applied uniformly on top of the aquifer and transferred to the receiving stream through steady-state groundwater flows, return flows and saturation excess overland flows. Without seepage, TTDs evolve from uniform- to power law-like- distributions depending on the average distance of the groundwater volume to the river. Remarkably, the coefficient of variation of the TTDs scales linearly with the groundwater volume to the river at any hillslope convergent/divergent rate in agreement with a theoretical prediction based on three analytical approximations. With seepage, the TTD progressively displays three separate modes corresponding (1) to the rapid saturation excess overland flows, (2) to the intermediary circulations ending up in seepage area and (3) to the slower circulations going from a recharge upstream the seepage zone to a discharge in the river. The coefficient of variation additionally depends on the extent of the seepage area.</p><p>Applied to a natural hillslope in the crystalline basement of Normandy (France), the same synthetic analysis demonstrates that the coefficient of variation is not only determined by the extent of the seepage zone but also by its structure in relation to the geomorphological local and global organizations. These results suggest the possibility to assess the variability of transit times by combining geomorphological analysis, surface soil saturation observations and environmental tracers.</p>

Revisiting SWAT as a Saturation-Excess Runoff Model

The Soil Water Assessment Tool (SWAT) is employed throughout the world to simulate watershed processes. A limitation of this model is that locations of saturation excess overland flow in hilly and mountainous regions with an impermeable layer at shallow depth cannot be simulated realistically. The objective of this research is to overcome this limitation with minor changes in the original SWAT code. The new approach is called SWAT-with-impervious-layers (SWAT-wil). Adaptations consisted of redefining the hillslope length, restricting downward percolation from the root zone, and redefining hydrologic response units (HRUs) such that they are associated with the landscape position. Finally, input parameters were chosen such that overland flow from variable saturated areas (VSAs) corresponds to the variable source interpretation of the Soil Conservation Service (SCS) curve number runoff equation. We tested the model for the Town Brook watershed in the Catskill Mountains. The results showed that the discharge calculated with SWAT-wil agreed with observed outflow and results simulated with the original SWAT and SWAT-hillslope (SWAT-HS) models that had a surface aquifer that transferred water between groups of HRUs. The locations of the periodically saturated runoff areas were predicted by SWAT-wil at the right locations. Current users can utilize the SWAT-wil approach for catchments where VSA hydrology predominates.

Multiple runoff processes and multiple thresholds control agricultural runoff generation

Hydrologic connectivity associated with runoff processes is a critical concept for understanding catchment hydrologic response at the event timescale. However, to date, most attention has focused on single runoff response types in individual research catchments. Here we examine how runoff response and the catchment threshold response to rainfall affect a suite of runoff generation mechanisms in a small agricultural catchment. A 1.37 ha hillslope in the Lang Lang River catchment, Victoria, Australia was instrumented and hourly data of rainfall, runoff, shallow groundwater level and isotope water samples were collected. We analyse 60 rainfall events that produced 38 runoff events over two runoff seasons. Our results show that the catchment hydrologic response was typically controlled by the antecedent soil moisture condition and rainfall characteristics. There was a strong seasonal effect in the antecedent moisture conditions that led to marked seasonal scale changes in runoff response. Analysis of shallow well data revealed that streamflows early in the runoff season were dominated primarily by saturation excess overland flow from the riparian area. As the runoff season progressed, the catchment soil water storage increased and the hillslope connected to the riparian area. The hillslope transferred a significant amount of water to the riparian zone during and following events. Then, during a particularly wet period, this connectivity to the riparian zone, and ultimately to the stream, persisted between events for a period of one month. These findings are supported by isotope results which showed the dominance of pre-event water, and increased contributions of new water early (rising limb and peak) in the event hydrograph for wetter conditions. We conclude that event runoff at this site is a combination of subsurface event flow and saturation excess overland flow. However, during high intensity rainfall events, flashy hillslope flow was observed even though the soil moisture threshold for activation of subsurface flow was not exceeded. We hypothesize that this was due to the activation of infiltration excess overland flow and/or fast lateral flow through preferential pathways on the hillslope and saturation overland flow from the riparian zone.