Saturday, 15 July 2006

Soil Water Content Patterns in the High Plateau of Sierra De Gador (Almeria, Se Spain). Implications for the Local Water Balance.

Yolanda Cantón1, Luís Villagarcía2, Albert Sole-Benet3, Francisco Domingo3, Juan Puigdefábregas3, and Sergio Contreras3. (1) Univ de Almería, La Cañada de San Urbano, Almeria, Spain, (2) Univ Pablo de Olavide, Carretera de Utrera, km 1, Sevilla, Spain, (3) E.E.Z.A./C.S.I.C., General Segura, 1, Almeria, Spain

In many Mediterranean coastal zones, especially in southeast Spain, water demand has increased due to expansion of tourism and agriculture with the consequent accentuation of the dependence on underground water resources. A paradigm is the Sierra de Gador-Campo de Dalias system, in SE Spain, where an intensive exploitation of the aquifer is occurring as a consequence of the intensive irrigation of 20,000 ha and the growing water demand from tourism. Knowledge of soil moisture dynamics is especially crucial in these areas, because of its important impact on groundwater resources, given the scarcity of rainfall and the high evapotranspiration rates. The variables involved in the local water balance at the high plateau, at 1,500 m a.s.l. in Sierra the Gador were monitored and analysed during an hydrological year, in order to determine the water drainage that might contribute to the aquifer recharge. The analysis of the spatial and temporal patterns of soil water content and their relations to other soil properties along a hillslope transect show a pronounced variability in short distances linked to the differences in soil and ground cover characteristics. Seasonal and daily scales reveal that the soil moisture regime get moister going downslope at soil depths of 0.06 m and 0.25 m as a consequence not only of the runon water contribution from the upper sector, but also because of the higher clay content of the soil at the lower sector. Soil water content remains higher at 0.25 m than at 0.06 m for the whole evaluated period not only by the high evaporation losses of the surface horizon but also by the higher clay content and higher organic matter content at depth. At event scale, in a few occasions, after rainfall events smaller than 1 mm an increase up to 3% in soil water content at 0.06 m was observed and explained by the role of cracks causing very local high infiltration rates. Nevertheless, 84% of rainfalls lower than 1 mm caused an increase in soil water content lower than 0.5% or no increase, and 47% of all rainfall events did not affect soil water content. Although there is a significant logarithmic relationship between rainfall depth and soil water content increase, it only absorbs 60% of the variance as a consequence of the influence of other variables (antecedent soil water content, rainfall intensity or crack pattern). At a soil depth of 25 cm, a higher rainfall is necessary to produce a slighter increase in soil water content. For the hydrological year 2003-04, the total deep drainage or percolation calculated as D = P - AET - Roff + Ron - ΔS (being P = precipitation, Ron = runon, Roff = runoff by overland flow or sub-surface flow, AET = actual evapotranspiration, ΔS = change in soil water storage) we obtained a D of 215 mm year-1 (42.4% of the total rainfall). The estimation of deep drainage considering only the water content (assuming no drainage when soil water content is lower than field capacity) provided very similar results but it was very sensible to the coarse fragment content of the soil which in the study area is very heterogeneous and represents more than 20% of total soil volume.

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