Graduate Thesis Or Dissertation
 

Application of geostatistics to regional evapotranspiration

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  • Regionalized variable analysis has been used to study a number of meteorological and hydrological variables including precipitation (Delhomme and Delfinier, 1973), streamfiow (Villeneuve et al., 1979) and hydraulics of groundwater aquifers (Gambolati and Volpi, 1979). In the research work reported in this thesis, an attempt was made to characterize the spatial variability of evapotranspiration over the state of Oregon using methods of Geostatistics. Application of the geostatistical concept of semivariograms was required to describe spatial variation of a variable as a function of distance. The semivariogram models were derived from reference evapotranspiration rates computed from data at 175 weather stations during May through September of 1979. The resulting semivariograms for all months but September were anisotropic and indicated that the rate of change in reference evapotranspiration was higher in the east-west direction than in north-south direction. The semivariograms were fitted by spherical models and applied to perform interpolation of evapotranspiration using the geostatistical technique of kriging. Kriging estimates of evapotranspiration were made at approximately 1,600 locations where no weather data existed, corresponding to corners of a 12.8 km by 12.8 km square grid system laid over the state of Oregon. Using computer plotting routines, the estimates were transformed into contour maps of evapotranspiration and of contour maps of kriging variance. The contour curves of reference evapotranspiration agreed with the general distribution of climatological and topographical features over the state of Oregon. A self-validation test performed on the semivariograms revealed the existence of a certain bias which could be removed by deriving individual semivariograms for climatic subregions within the state. The results of this research indicate that the spatial variability of reference evapotranspiration over large geographical areas can be described by semivariogram models and this information can be used to predict evapotranspiration rates by means of kriging. Such a procedure could be effectively applied to increase the efficiency of water resources utilization in the arid regions.
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