Mapping trends in water table depths in a Brazilian Cerrado area

Rodrigo Lilla Manzione 1, Martin Knotters 2 and Gerard B. M. Heuvelink 2
1 INPE (National Institute for Spatial Research) – DPI (Image Processing Division)
Avenida dos Astronautas, 1758. Caixa Postal 515, 12227-010, São José dos Campos, SP, Brazil
Tel.: + 055 12 39456481 
2 Alterra – Soil Science Centre
Droevendaalsesteeg 3, 6708PB Wageningen, The Netherlands
Tel.: + 031 317 474240, 

The Cerrado region is the most extensive woodland-savannah in South America, situated at the central Brazilian Plateau and characterized by wet and dry periods well defined during the year. During the past 30 years, the original vegetation has been replaced by extensive cattle fields and agricultural crops, which are less adapted to drought than the Cerrado vegetation. Therefore, irrigation is increasingly applied, resulting in changes of the hydrological system. The aim of this study is to map systematic  changes of the water table depths, in order to indicate areas with potential risks of future water shortage. To this purpose the PIRFICT- model is applied, a transfer function-noise (TFN) model with a Predefined Impulse Response Function In Continuous Time. Being the most important driving forces of water table fluctuation, precipitation and evapotranspiration are incorporated as exogenous variables into the model. Besides, a linear trend component is incorporated, reflecting systematic changes of water table depths over time. The linear trend parameter of the  time series model is interpolated spatially using universal kriging, by utilizing  actual land use derived from Landsat images as ancillary information. Series of 30 months length of semi-monthly observed water table depths are available from 40 wells in the Jardim river watershed. In this area almost all natural Cerrado vegetation has been replaced by agricultural crops, some of which are intensively irrigated. The time series models are calibrated to the 40 series, and next the trend parameter reflecting systematic changes of water table depths is mapped. The resulting map indicates potential risks of water shortage. The kriged trend parameter is evaluated by cross-validation. The significance of the interpolated trend parameter is also mapped. The uncertainties associated with  the mapped trend parameter are large, suggesting that longer time series of water table depths are needed to obtain more accurate results.

Keywords: groundwater levels, phreatic levels, time series modeling, spatio-temporal modeling, trend analysis

In: Caetano, M. and Painho, M. (eds). Proceedings of the 7th International Symposium on Spatial Accuracy Assessment in Natural Resources and Environmental Sciences, 5 – 7 July 2006, Lisboa, Instituto Geográfico Português

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