determination the crop coefficient and water requirement of lentil, using water balance method (case study: khorram abad)

introduction evapotranspiration is one of the most important elements of hydrological cycle which is required for water resource management and design of irrigation systems. crop evapotranspiration (etc) can be measured by the lysimeter using water balance equation. it also can be estimated by multiplying reference evapotranspiration (eto) and crop coefficient (kc). considering the importance of this parameter, numerous studies have been performed to determine the kc of major crops in each region. lentil is one of the legumes which is cultivated mainly in lorestan province. although this crop is the second in terms of the area under cultivation among the legumes in the country, the water requirement and crop coefficients of this crop is not determined in lorestan province yet. the main purpose of the current study is to determinate lentil water requirement and its kc in khorram abad. materials and methods a lysimetric experiment was conducted during the lentil growth season in 2013 at the research farm of agricultural faculty, khorram abad, lorestan university. the farm is located in a cold semi-arid climate. physical and chemical properties of the soil in the lysimeters were determined in the laboratory. the average of soil moisture content in the lysimeter at the field capacity and permanent wilting point was determined as 34.4 and 24.8 volumetric percentage respectively. in this study, lentil (gachsaran cultivar) and grass were cultivated each on four micro drainable lysimeter with the diameter of 48 cm and height of 80 cm. maximum allowable depletion (mad) of soil moisture was considered as 30% to ensure that crops are not faced with water stress. length of the crop growth stages (initial, development, mid-season and late-season) was determined by daily observation of the field and measuring crop canopy. lentil and grass evapotranspiration was determined using water balance method. the evapotranspiration from grass was considered as reference evapotranspiration (eto). the measured values of eto and lentil evapotranspiration was compared with the calculated values by cropwat, ref-et and agwat (basis of national document of water) packages. crop coefficient (kc) was calculated as the ratio of lentil evapotranspiration over eto. average of kc for each stage was also calculated and compared with literature. results and discussion the average of reference crop evapotranspiration (eto) was increasing thorough the growth period of lentil from 3.7 to 8.6 mm.day-1. this was due to increasing of sunshine hours and air temperature during that period. total amount of lentil and grass evapotranspiration during the growing period of lentil was determined as 476 and 568 mm respectively. total amount of calculated eto in this period by cropwat, ref-et and agwat packages was 551, 566 and 340 mm respectively. results indicated that calculated values of eto by cropwat and ref-et is almost equals to measured eto. however, agwat also uses penman-monteith equation but its results were far from the measured values of eto because of differences in weather data. results also indicated that lentil evapotranspiration was low at the initial stage, increased during the development stage and decreased during the late season. the maximum crop evapotranspiration of 88.46 mm per decade was measured at the period of 16-25 may 2013. measured lentil evapotranspiration in the current study was higher than values calculated by agwat software package for this region. lengths of initial, development, mid-season and late-season stages were determined as 19, 20, 33 and 24 days respectively. the average of lentil crop coefficient for initial, development, mid-season and late-season stages was calculated as 0.45, 0.89, 1.19 and 0.56 respectively. calculated kc values in this study were compared with the values of kc in fao 56 report and agwat. calculated kc at initial and mid stages (0.45 and 1.19 respectively) were close to values reported in fao 56 (0.4 and 1.1 respectively) but far from agwat (0.15 and 1.05 respectively). in general, calculated crop coefficients in the current study were higher than the values presented in fao 56 and agwat package software. measured values of lentil evapotranspiration were higher than values reported in agwat. the reason was underestimation of eto by agwat and lower values of kc for lentil. conclusions as there was no measurement of lentil evapotranspiration and crop coefficient, the results of current study can be used as a base for further studies about this issue in the region. comparison of measured values of eto and lentil crop evapotranspiration with the values calculated by agwat package indicated the under estimation of both parameters by agwat. however, it is recommended to do more studies on this subject but the results of current study can be used to determine lentil evapotranspiration instead of values reported in agwat and national documents of water for lorestan province.