Comparison of Penman- Monteith FAO Method and a Class Pan Evaporation with Lysimeter Measurements in Estimation of Rice Evapotranspiration in Amol Region



Rice provides more than 80 percent of the calories consumed in a large part of Asia. Rice, in addition to the food, the dependence of the water has unique characteristics. Supplying water requirements of rice, as a plant with high nutritional and economic value is important in many paddy lands. Evapotranspiration for each product such as rice is the ratio of the reference crop evapotranspiration. This ratio is called the crop coefficient that is variable due to the physiological and morphological differences between plant species, and climatic characteristics of the region. With increasing duration of growth, Crop coefficient increases, so that, at crop development, to the maximum and then decreased. In this study, the evapotranspiration of rice plants based on climatic data using pan evaporation and Penman Mantys, compared with the values lysimeter evapotranspiration, was estimated.

This study was conducted during two years of field research and development center to promote technology Haraz at the km 10 Amol - Mahmoud Abad road. In terms of climate, long-term average maximum temperature 32.7 oC, minimum temperature 7.8 oC and mean annual precipitation 882.6 mm that 63 percent of precipitationoccurs in the month of September to December. Two-year field study was performed to investigate paddy rice crop evapotranspiration (ETc) wfor two types of paddy rice variant, Khazar and Tarem, by direct method (lysimeter) and calculated method of FAO in Amol, Mazandaran state. To determine the water requirement and crop coefficient of rice plants, the improved cultivars (Tarom and Khazar) as high yielding varieties during the growing season and crops were compared. All measurements of the water requirements of rice, in machine-based stations Laysymtr Ghyrvzny length 1 m, width 1 m and height was 65 cm. simultaneously reference evapotranspiration (ETo) was estimated by daily measured A class evaporation pan data after calibration of pan coefficient. Calibrated pan coefficients were calculated by different equations such as Cuenca, Allen, Pruitt, Snyder, modified Snyder, Orang and FAO. To determine the accuracy of calculating evapotranspiration rice, measurements of Laysymtry were used. Laysymtry values were used as reference data for comparing the calculated values.

Results and Discussion
Obtained results by lysimeter showed, rice evapotranspiration in the first and second year of 578.5 (Tarom variety) and 481.6 mm (Khazar variety), respectively. This is due to a higher being during growthof Tarom variety in the first year than Khazar variety in the second year. However, a Tarom variety is long-term with tall plants. Increased during growth, and tall plant height, is effective in the amount of water. The maximum amount of KC in the third stage of rice plant growth, for both years was 1.2. The minimum in the fourth stage, for both first and second years was 0.9. In the present study for both farming year, the value of Kc in the initial stage of growth, was1.09 and 1.13, respectively.
Two-year results showed that FAO method overestimate calculated crop evapotranspiration values by 4-5% in comparison of lysimeter measurements. Results showed that Eshnaider method is suitable for ETo estimation of pan coefficient and evaporation pan estimated ETo well in the study region. Estimation of ETc on the basis of pan evaporation data, Eshnyder pan coefficient and FAO modified crop coefficient showed only on the average 1.5% underestimation in comparison with actual ET (ETc). Therefore this method is suitable and practical in the study region. Overall, theresults showed taht using appropriate methods for estimating reference crop evapotranspiration and the crop coefficient, evapotranspiration of rice can be calculated with good accuracy. Applications and is suitable for different regions.

In this study, due to economic importance and nutritional value of rice in Iran, two-year study to determine the water requirement and select the appropriate method for estimating the evapotranspiration of rice was done. Obtained results by lysimeter showed, rice evapotranspiration in the first and second year of 578.5 and 481.6 mm, respectively. Comparison of rice plant evapotranspiration estimates showed that with an estimate of reference crop evapotranspiration and crop coefficients, the results are closer to the actual value. Among the various methods of estimating pan coefficient, Schneider was more appropriate than other methods. It revealed that, using appropriate methods to estimate reference crop evapotranspiration and crop coefficients applied, can calculated rice evapotranspiration with good accuracy that is applicable and appropriate for different regions.