Temporal and spatial processes controlling the duff water budget. Experimental and model evidence
Creators: name: Keith, David
Publisher: name: University of Calgary
Year: 2009

description: Bibliography: p. 144-153
description: The forest floor in many ecosystems consists of a partially decomposed organic layer (duff). The duff, along with the litter layer, is the boundary between the atmosphere and the mineral soil and it therefore plays a crucial role in forest hydrology. The duff shows major seasonal changes in moisture with rapid drying during wet periods shortly after a precipitation event; the duff moisture content decreases exponentially within 24 hours of a precipitation event. During periods with no precipitation the drying of the duff is much slower than during the rapid drying period, and a diurnal pattern of drying becomes evident. Our objective in Chapter II is to better understand the processes controlling the duff water budget during dry periods using modeling and field manipulations. Field results indicate that during dry periods the moisture content of the top duff layer cycles diurnally with minimal moisture movement between the duff and mineral soil. During these periods the moisture content peaks in the early afternoon, near the time of maximum evaporative demand. These cycles are controlled by diurnal meteorological fluxes that drive coupled liquid and vapor fluxes within the duff. The model couples heat and mass transfer with meteorological fluxes to quantify the duff budget. The model reproduced duff drying patterns with Nash-Sutcliffe efficiencies and Coefficients of Determination greater than 0.910 and 0.970 respectively. Wavelet analysis indicated that the model results were highly correlated with the actual drying pattern throughout both simulation periods. The model results confirm the results of the field manipulations, meteorological fluxes lead to diurnal cycles in moisture content in the duff F layer, and these processes are driven by the interaction of liquid and vapor fluxes within the duff. In Chapter III the focus is on the processes that lead to spatial variability in the duff water budget. In conjunction with empirical evidence a three dimensional water budget is used to quantify the processes leading to variability in duff moisture at the canopy and hillslope scales. Topographic influences are found to lead to hillslope scale variability in the duff moisture content throughout the study period. The water budget on the poorly developed thin duff layer found on a Pine hillslope is controlled by the spatial variability in solar radiation (i.e. an atmospheric process), while on the Spruce hillslope, with a thicker well developed duff layer, rapid redistribution through the duff (i.e. a hillslope process) during and after rainfall leads to the bottom of the hillslope having higher moisture content. The rapid redistribution of moisture through the duff occurs within 24 hours of the termination of rainfall. During this transient period the lateral redistribution of moisture through the duff layer occurs primarily through macropores in the duff. This lateral redistribution occurs over relatively short distances, meters, and is equivalent to the length of several large hydrologically connected macropores. Redistribution through these macropores results in higher duff moisture content on convergent parts of the hillslope. The hillslope scale pattern is due to more convergent regions located near the bottom of a hillslope than near the top.