بررسي ضرايب تشتك تبخير و تعيين ضريب گياهي از مزارع چمن و يونجه در ايستگاه كشاورزي كركج دانشگاه تبريز

Investigation of pan - and determination of plant - coefficients from grass and alfalfa fields in the Karkaj agricultural station of Tabriz University

In order to study the effects of field conditions on pan coefficient (KP), three classes a pans were installed at a grass field according to FAO standards, and in an alfalfa field at 50 and 80cm elevation. Weekly evapotranspiration from grass (ET0) were also computed using water balance equation. Evaporation from the three pans (Epg, Ep50 and Ep80 ) was measured every day. Comparing weekly average values of pan coefficients from alfalfa field (Kpg) and Kp values computed by FAO method indicated that this method under-predicts the pan coefficients by about 12%. Comparing Kp50 and Kp80 values indicated significant variation between them, whereas the variation between Kpg and Kp50 values was not significant. Because of the similarity of Kp50 to Kpg values during most of the weekly periods and their average, one may use class A pan records installed at an alfalfa field to computer ET0by applying Kp50 values as determined in this investigation. In this study crop coefficients (Kc) of alfalfa were determined by using evapotranspiration amounts. Several empirical equations were also used for calculation of evapotraspiration to compare with the results obtained by the water balance and evaporation pan methods.