Analysis of evapotranspiration for various climatic regimes using geostatistics Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/12579w05z

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  • Research was conducted to determine the applicability of using the theory of regionalized variables or geostatistics in characterizing the spatial variability of reference evapotranspiration (ETr) over various climatic regimes for the state of Oregon. The state was divided into five climatic regions based on topographic features and local meteorological conditions: 1) Coast, 2) Willamette Valley, 3) Southwestern Valley, 4) North East, and 5) South East. The local ETr estimates provided from the FAO-modified Blaney Criddle method were divided into their respective regions and averaged over the three years. The variogram analysis was performed on the average ETr estimates for May through September in each region. Model variograms were fitted to the calculated sample variograms and a cross validation routine performed to test the chosen model. For four out of the five subregions, a verified spherical model was obtained for the individual months. Difficulties related to the low number of samples representing the region made it difficult to confirm model variograms in region 3. The model variograms determined for the average ETr estimates were used as the model variograms for the individual years (1985, 1986, 1987) to test the hypothesis that one model variogram could represent the spatial correlation of ETr for the region. Due to the inconsistencies seen in the analysis, no valid conclusions could be made to support this hypothesis. It is recommended that developing the model variograms from long term average ETr estimates instead of the short three year average could provide better results when applied to individual years. Kriging was performed on the average ETr estimates for July and September in regions 1, 2, 4, and 5. From the kriging analysis, estimates of ETr along with the standard deviation over a grid representing the region were produced. Contour maps were plotted using the gridded information of kriged ETr and standard deviation of the kriged estimates. The results of these contour maps proved to be a good representation of the ETr estimates within most of the regions. It was noticed that the values of ETr of individual meteorological stations within the regions influenced the shape of the contour lines and the shape did not necessarily correspond to topographic effects in Oregon. One explanation is that the weather stations used for the ETr estimates are generally representative of the valleys in each region. For use in hydrologic modeling or irrigation system design and scheduling in valleys, the kriged ETr estimates could be very satisfactory. However, for use in large scale hydrologic modeling or global circulation models, a method to account for the topographic effects must be included in the kriged ETr estimates. A method that might prove successful is developing a spatial correlation between ETr and elevation through a geostatistical technique termed cokriging. Another problem with the regional analysis was comparison at regional boundaries. To effectively utilize these contour maps for the whole state, there must be some method to deal with the transition zones between regions. A possibility is to combine kriged estimates from each region into one file representing the whole state and producing contour plots for the overall data file. The use of geostatistics is becoming more common in hydrology and its use is expected to grow. From this work, geostatistics proved to be a possible tool to generate estimates and computerized plots of reference evapotranspiration (ETr). However, there are difficulties to overcome for geostatistics to be applied operationally in estimating regional evapotranspiration.
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