Runoff-generating processes in hydromorphic soils on a plot scale: free gravity-driven versus pressure-controlled flow

Draining soil water is an important runoff generator. This study aims to describe runoff-generating processes on a plot scale (1 m(2)) in hydromorphic soils with different initial soil water contents. We irrigated 16 hydromorphic soils in the northern Pre-Alps in Switzerland and recorded the variations in water content with time domain reflectometry (TDR) at five different depths per plot. Sprinkling was repeated three times at approximately 23-h intervals and lasted for 1 h with a volume flux density of 70 mm h(-1). The comparison between the measured water content of the drainages with two physically based models revealed which of the flow processes dominated during water recessions. We distinguished between vertical drainage, lateral outflow and infiltration without drainage. Approximately 45% of all recorded time series of soil water content did not drain within approximately 20 h after the end of irrigation, about 25% drained laterally and 10% of the outflow was vertical. The drainage of the remaining 20% was the result of both lateral and vertical water flow (approximate to 12%), or was not interpretable with the approaches applied (approximate to 8%). Vertical flow was only observed in layers without any or with just a few hydromorphic features. Lateral draining horizons had approximately half the storage capacity and amplitude of water recession of those with vertical flow. Vertical flow was only observed in the upper soil horizons. Thus, vertical flow transmitted water to layers with lateral outflow and did not delay runoff by deep percolation. Increasing initial soil moisture increased the percentage of water content recordings according to a lateral outflow slightly, while vertical flow was less frequent. Copyright (C) 2010 John Wiley & Sons, Ltd.