Abstract:The study is focused on runoff formation processes at two scales: the scale of a small mountainous catchment at its outlet and the scale of an experimental plot located in a typical hillslope subregion. The heterogeneous soil profile of the catchments is formed by Cambisols developed on granite bedrocks. The surface runoff appears rarely, the subsurface flow forms a dominant part of the hydrograph. From the period 1998-2008, a set of 44 rainfall-runoff episodes was selected to analyse the rainfall-runoff relationship using multiple regression analysis. From a set of physical parameters, the initial soil water content came out as the statistically significant parameter that controls the runoff forming process at the catchment scale. The rainfall-runoff relationship at the experimental plot scale is more scattered. The dynamic thresholds of rainfall totals apparently control the ratio and the magnitude of stormflow at both scales. Up to the threshold value, the runoff strongly depends on the initial saturation conditions. Above the threshold value, the initial soil moisture conditions are less important.

Since Hewlett and Hibbert's publication in 1967, there has been a slow recognition that saturated excess runoff is the main runoff mechanism in vegetated watersheds. Yet, most pedotransfer functions for predicting runoff are based on infiltration excess runoff. We, therefore, developed a simple pedotransfer function to predict saturation excess runoff, using data from eight watersheds on three continents. The runoff response was very similar for all watersheds, despite differences in climate, size, topography, and land use. Direct storm runoff occurred after a threshold amount of rainfall was exceeded. Runoff was linearly related to rainfall depth, indicating that a nearly constant proportion of the watershed was the source area. Size of source areas decreased with increasing depths of soils. The rainfall threshold was strongly dependent on the initial moisture conditions. The developed pedotransfer function for saturation excess runoff was used to predict water level fluctuation of two terminal lakes on the Caribbean Islands over a 25‐ to 30‐yr period with the rainfall threshold computed following Thornthwaite–Mather and baseflow from the remaining part of the watershed employing a linear reservoir model. Taking the simplicity of the prediction technique with only four calibrated parameters into account, lake levels were predicted reasonably well to very good, including the rise in the lake level in the last 10 yr when the climate in the region became wetter. It is expected that the linear relationship of rainfall and runoff holds for storms lasting several days and can simplify flood predictions.

Abstract:In agricultural lands has the soil moisture uptake from the root system a significant effect on the water regime of the soil profile. In texturally heavy soils, where preferential pathways are present, infiltrated precipitation and irrigation water with diluted fertilizers quickly penetrate to a significant depth and often reach an under-root zone or even the groundwater level. Such a scenario is likely to happen during long summer periods without rain followed by heavy precipitation events, when a part of the water may flow through desiccated cracks. Since 2001 the effects of drip irrigation and nitrogen fertilization of potatoes (Solanum tuberosum L., cultivar Agria) have been monitored within the frame of a research project at the experimental site Valecov (Czech Republic). Based upon the measured data an attempt has been made to simulate the water regime of the soil profile at a selected experimental plot, considering the impact of preferential flow and root water uptake. The dual-permeability simulation model S 1D Dual (VOGEL et al., 2000) was used for the simulation. The soil hydraulic parameters were inversely determined using LevenbergMarquardt method. Measured and simulated pressure heads were utilized in the optimization criterion. The scaling approach was applied to simplify the description of the spatial variability of the soil profile.The results of simulations demonstrate that during particular rainfall events the water reaches significant depths of the soil profile via preferential pathways. The effect of the root zone is dominant during dry periods, when capillary water uptake from the layers below roots becomes important. This should be taken in account into the optimization of the drip irrigation and nitrogen fertilization schedule.