A Reserach on calibration of some amprical equations for grass reference evapotranspiration under field and small lysimeter conditions in Antalya region

ABSTRACT A RESEARCH ON CALIBRATION OF SOME AMPRICAL EQUATIONS FOR GRASS REFERENCE EVAPOTRANSPIRATION UNDER FIELD AND SMALL LYSIMETER CONDITIONS IN ANTALYA REGION Koksal AYDINŞAKİR M. Sc. Farm Structures and Irrigation Department Supervisor: Prof. Dr. Ruhi BAŞTU? June 2001, 96 Pages In this study, it was aimed at calibrating some amprieal equations of grass reference evapotranspiration under field and lysimeter conditions in Antalya region. To achieve the purpose, actual evapotranspiration values have been measured and compared to calculated reference evapotranspiration values to find the best method for estimating evapotranspiration. In the study, Penman-Monteith and FAO versions of Blaney- Criddle, Radiation, Class A Pan and Penman equations were used in order to compute grass reference evapotranspiration utilizing climatic data. The study was carried out an experimental plot size of 1 8 x 60 m has been set up to measure actual grass reference evapotranspiration under field conditions and in three containers, as lysimeter, having 20 cm of diameter and 40 cm of height. The climatic data used to compute reference evapotranspiration were obtained from climatic station which was located near the experimental plot and Antalya Meteorology Agency. The experimental plot was irrigated by sprinkler irrigation, while the containers were irrigated by a measured cup. During the study, the water content in the soil profile was monitored gravimetrically and by tensiometers in three replications, while it was only IVmonitored in one container by Tensior 5 tensiometer in lysimeter. The soil water content was kept around field capacity both field and lysimeters. During the experiment, the grass was cut 9 times in total by a mower under field condition and by scissors in lysimeter when its height reached 10-12 cm. The height of grass was 4-5 cm after the cut. Seasonally, 1405 mm irrigation water has been applied with 53 irrigation events. The irrigation interval was aproximetely 3 days, althought it changed from month to month. Seasonal irrigation water that has been applied to the containers were 1635.6 mm. The irrigation interval in the containers were 3-4 days. The number of irrigations for lysimeter were 30. Seasonal, monthly, 10-day and daily actual evapotranspiration under field and lysimeter conditions were differed from based on climatic data. The daily average evapotranspiration in 10 day period ranged between 8.5 and 12.0 mm/day unfder field conditions, while it ranged between 9.2 and 14.4 mm/day under lysimeter conditions. The reference evapotranspiration calculated by the equations based on climatic data ranged between 4.4 and 1 1.4 mm/day. The actual evapotranspiration under field and lysimeter conditions during the study and the reference evapotranspiration obtained from the different equations were evaluated by a statistical approach and regression equations were obtained. The equation which had the lowest root mean squares, the highest correlation coefficient and the highest ratio of reference evapotranspiration to actual evapotranspiration was considered the best equation for representing actual evapotranspiration under field and lysimeter conditions. According to the above mentioned criteria, Class A Pan was chosen the best equation for representing field conditions. The values for above mentioned statistics for Class A Pan were found as follows: Root Mean Square Error (RMS) 1.96; corelation coefficient (r2) 0.679 and the ratio of evapotranspiration for Class A Pan to actual evapotranspiration (%Et) % 86. After the statistical analysis, the functional dependency of actual evapotranspiration on evapotranspiration measured byClass A Pan for Antalya region was found to be as ET = O.OOlETo2 + 0.41ETo + 6.28. Penman method also gave similar results (RMS=1.97, r2=0.446 and %Et=84). This is followed by Blaney-Criddle, Radiation and Penman-Monteith under field conditions. The most reliable method found as a result of statistical analysis under lysimeter condition for representing actual evapotranspiration was Class A Pan, as in the case of field conditions. In this method, the RMS, r2 and %Et were found to be 2.1, 0.709 and %84.79, respectively. The equation suggested to use was as ET =0.0775ETo2 - 0.69ETo + 10.2. The second equation which gave good results was the Penman method (RMS=2.2, 1^=0.366 and %Et=87.78). This is followed by Blaney-Criddle, Radiation and Penman-Monteith. Additionally, the actual evapotranspiration values obtained from lysimeters and field were plotted in the same graph and analyzed statistically. It was concluded that, based on t-test results, small lysimeters were not able to represent field conditions. According to the results, Class A Pan method can be suggested for calculating grass reference evapotranspiration in Antalya region. This is followed by Penman, Blaney- Criddle, Radiation and Penman-Monteith. Also, it can be suggested that, small lysimeters should not be used under field conditions. KEYWORDS: Grass, small lysimeter, evapotranspiration, reference evapotranspiration equations. COMMITTEE: Prof. Dr. Ruhi BAŞTU? Prof. Dr. Feridun HAKGÖREN AssocProf. Dr. Sadık ÇAKMAKÇI VI