Saturday, 15 July 2006

Effects on the Local Water Balance of Daily Soil Water Content Fluctuations in Semi-arid SE Spain.

Luís Villagarcía1, Ana Were2, Yolanda Cantón3, Francisco Fernández2, María José Moro4, Albert Sole-Benet2, and Francisco Domingo2. (1) Univ Pablo de Olavide, Carretera de Utrera, km 1, Sevilla, Spain, (2) Consejo Superior de Investigaciones Científicas, General Segura, 1, Almeria, Spain, (3) Univ de Almería, La Cañada de San Urbano, Almeria, Spain, (4) Univ de Alicante, Alicante, Spain

The recent development of non-destructive soil moisture probes capable of continuous recording has facilitated the detection of processes that were previously not well understood in assessing the temporal and spatial distribution of soil surface water content. The analysis of the temporal evolution of soil moisture (θ) has allowed the observation of daily fluctuations of soil water content, mainly in arid and semiarid conditions. Although descriptions of these fluctuations in θ are not new, the processes driving them are not well known. To explain θ oscillations, four main hypothesis have been considered in the literature: a) sensor measurements are influenced by temperature; b) water vapour adsorption from the atmosphere; c) thermally induced changes in soil pressure, e.g. dilatancy, and d) upward movement of water during evaporation from the soil surface. Recent research advances seem to demonstrate that b) is the main cause to explain the soil water fluctuations. This work aims to analyse θ fluctuations in semiarid areas with sparse vegetation under different soil cover and depths and to ascertain the processes driving these daily θ fluctuations and its importance in the local water balance. In one study area, located in Rambla Honda, a dry valley on the southern slope of Los Filabres range (Almería, SE Spain), soil water content (θ) was continuously monitored every 30 minutes in different positions: 1) under annual plants, 2) under bare soil, 3) under Retama sphaerocarpa and 4) under a transitional area, bare soil - Retama. Fluctuations were observed mainly within the θ interval between 5% and 15%. The different positions exhibited different amplitudes in their fluctuations, decreasing in the following order: under bare soil, under annual plants, under a transitional area (bare soil – Retama bush), under Retama bush, coinciding with the order of the amplitude of daily fluctuations in soil temperature and explained by the smoothing role of plant cover on microclimatic changes and consequently on θ changes. The daily θ fluctuation amplitudes were also affected by the soil water content, being higher in the θ interval 5% - 10 %. The amplitudes of daily fluctuations in θ decreased with depth, as did soil temperature. A lag in the θ sinusoidal curves with depth was also observed. The effects of microclimatic variables on θ daily fluctuations were analysed. The found patterns of soil water content fluctuations were compared with those recorded in a nearby badland area under different soil surface conditions (different types of soil crust and plant cover). There the amplitudes of the daily oscillations in θ were more pronounced, especially under lichen crusts and bare crusted soil (structural crust) and the fluctuation peaks changed with time. Experiments on the effects of different types of soil crust on θ daily fluctuations were performed. Finally, in both locations, the contribution of the θ daily fluctuations to available soil water was analysed: it represented a 60% average of the evapotranspiration for the studied period.

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