A case study on the use of appropriate surrogates for antecedent moisture conditions (AMCs)

Abstract

Abstract.While a large number of non-linear hillslope and catchment rainfall-runoff responses have been attributed to the temporal variability in antecedent moisture conditions (AMCs), two problems emerge: (1) the difficulty of measuring AMCs, and (2) the absence of explicit guidelines for the choice of surrogates or proxies for AMCs. This paper aims at determining whether or not multiple surrogates for AMCs should be used in order not to bias our understanding of a system hydrological behaviour. We worked in a small forested catchment, the Hermine, where soil moisture has been measured at 121 different locations at four depths on 16 occasions. Without making any assumption on active processes, we used various linear and nonlinear regression models (i.e. linear, quadratic, cubic, exponential, logarithmic and logistic) to evaluate the point-scale temporal relations between actual soil moisture contents and selected meteorologicalbased surrogates for AMCs. We then mapped the nature of the "best fit" model to identify (1) spatial clusters of soil moisture monitoring sites whose hydrological behaviour was similar, and (2) potential topographic influences on these behaviours. Two conclusions stood out. Firstly, it was shown that the sole reference to AMCs indices traditionally used in catchment hydrology, namely antecedent rainfall amounts summed over periods of seven or ten days, would have led to an incomplete understanding of the Hermine catchment dynamics. Secondly, the relationships between point-scale soil moisture content and surrogates for AMCs were not spatially homogeneous, thus revealing a mosaic of linear and nonlinear catchment "active" and "contributing" sources whose locations were seldom controlled by surface terrain attributes or the topography of a soil-confining layer interface. These Correspondence to: G. A. Ali (genevieve.ali@umontreal.ca) results represent a step forward for the Hermine catchment as they point towards depth-specific processes and spatiallyvariable triggering conditions that are not controlled by topography. Further investigations are, however, necessary in order to derive general guidelines for the choice of the best surrogates for AMCs in a catchment.

Antecedent Moisture Controls on Stream Nitrate Flux in an Agricultural Watershed

“…Pre-event conditions influence the connectivity of subsurface hydrologic flow pathways contributing water to streams as well as the distribution and availability of nutrients for leaching (Soulsby, 1995;Biron et al, 1999;Stieglitz et al, 2003;Freeman et al, 2007). Under wet antecedent conditions, catchments are in a spatially connected or active state where pre-event water contributions may dominate (Grayson et al, 1997;Ali and Roy, 2010). Under dry antecedent conditions, the catchment is in a disconnected or disorganized state that promotes new water infiltration (Grayson et al, 1997;Ali and Roy, 2010).…”

“…Under wet antecedent conditions, catchments are in a spatially connected or active state where pre-event water contributions may dominate (Grayson et al, 1997;Ali and Roy, 2010). Under dry antecedent conditions, the catchment is in a disconnected or disorganized state that promotes new water infiltration (Grayson et al, 1997;Ali and Roy, 2010). The quantitative measure of hydrologic connectivity and antecedent moisture conditions is complicated, requiring the use of surrogates or proxies (see review by Ali and Roy [2010]).…”

“…Under dry antecedent conditions, the catchment is in a disconnected or disorganized state that promotes new water infiltration (Grayson et al, 1997;Ali and Roy, 2010). The quantitative measure of hydrologic connectivity and antecedent moisture conditions is complicated, requiring the use of surrogates or proxies (see review by Ali and Roy [2010]). Further, the magnitude and dynamics of nitrate-N flux associated with differing antecedent moisture conditions over variable timescales and spatial scales is not well understood, limiting our ability to predict nitrate-N flux and load response to rainfall events.…”

“…Increased hydrological connectivity during wet cycles means that precipitation events rapidly mobilize water to the stream, with both the baseflow contribution and overland flow diluting the in-stream C. The higher degree of spatial subsurface connectivity may result in a dominant pre-event water contribution (Grayson et al, 1997;Ali and Roy, 2010). Flux of nitrate-N is high due to excess groundwater released to the stream (Iqbal, 2002) and encountering the mobile nitrate-N pool.…”

An assessment of vegetation characteristics and hydrologic flow pathways on the effectiveness of vegetated buffer strips for phosphorus reduction in an agricultural watershed

“…AMCs are difficult to quantify because there is no single parameter that can be measured. Combining site specific data from a number of surrogates including subsurface topography, precipitation, soil moisture, groundwater level and vegetative cover is the best way to describe AMCs for a specific watershed (Ali & Roy, 2010). In areas where subsurface pathways did not previously exist, they begin to emerge as the vadose zone becomes saturated.…”