برآورد ضریب گیاهی نخود با استفاده از لایسیمتر در منطقه خرم‌آباد

In recent years, Iran has been facing severe droughts and water shortages, therefore crop water requirement determination is an important issue in designing and managing plans for coping drought condition and optimum operation of irrigation systems. Accurate estimation of evapotranspiration could provide enough crop water requirement and prevent water losses. One of the most accurate methods to measure evapotranspiration is using Lysimeter. Lorestan province and Khorramabad region have major share of chickpea (Cicerarietinum L.) production in Iran. The Chickpea is the most important crop in Lorestan province, which has more cultivation area compare to the other crops. However, so far no comprehensive research has been conducted to estimate the chickpea water requirement. Therefore, the aim of this research is estimating water requirement and different crop coefficients of chickpea in Khorramabad region.
This research was conducted in research farm of Lorestan Agriculture Faculty with a height of 1148 meters above sea level and cold semi-arid climate during 2013 to find chickpea (Cicerarietinum L.) water requirement and different crop coefficients. To achieve these aims, 4 drainable Lysimeters were selected that their diameter and height were 0.45 and 0.8 m, respectively. The crop density was 50 plants per m^2. These Lysimeters were made from polyethylene. Soil texture was silty loam. The soil moisture capacity was measured by a pressure plate. Soil moisture in the field capacity was 26.66 Wt% and at the permanent wilting point was 19.24 Wt%. Soil bulk density was also 1.289 gr/?cm?^3. In this research, the time and the volume of each irrigation was chosen regarding to soil moisture less than 70% of FC in which the crop is not exposed to any moisture stress. In order to provide drainage in each Lysimeter, 15 cm from the Lysimeter bottom was filled with coarse sand and the rest depth of the Lysimeter was filled with the mixed of soil and fertilizer. To provide the same growth conditions, Lysimeters were placed within a chickpea farm. The excess water of each Lysimeter was drained and measured using a calibrated container. The soil moisture before irrigation at depths of 30 and 60 cm were measured with a moisture meter (IDRG) using installed sensors in each Lysimeter. Evapotranspiration was calculated using the water balance equation. According to this principal, amount of moisture in the soil equal to the amount of water minus the drainage water. The potential evapotranspiration was determined at the same time using the four grass planted Lysimeters. By dividing the chickpea evapotranspiration by potential evapotranspiration, crop coefficients were obtained. The crop coefficient value was not fixed and changed during the period of vegetative growth. According to the proposed method of FAO, for the period of vegetative growth curve was plotted and obtained crop coefficient was applied to each growth stage. In this method, it is assumed that the crop coefficient is changing during plant growth from the beginning to harvest stage. Although this change is not linear, but the vegetative period is divided into four stages. At each stage of plant growth, this change happens in the specified line. With daily meteorological data (T_max, T_min, ?RH?_max, ?Rh?_min, U_2 and n) and FAO equation in this period, potential evapotranspiration was measured and the results were compared.
According to the results, the water requirement of chickpea was estimated equal to 438 mm. The potential evapotranspiration also was estimated equal to 554.2 mm. 10-day minimum and maximum evapotranspiration of chickpea were 80.35 and 19.44 mm/10days, respectively, which these were in the months of June and March. The average crop coefficient was 0.488 in the initial growth stages, leaf area index and transpiration rate by increasing plant growth and during development stage, the crop coefficient increased and reached to 0.955. In the middle stage of the biological activity of the plant, crop coefficient was increased to 1.083 that creating a constant curve for this period. For the final stage, crop coefficient of 0.37 was also obtained. At the end of the growing season and completing the ripening stage, the chickpea grain was harvested. The chickpea water requirement for Khorramabad region was estimated at 338 mm using Iranian Water Requirement Document whereas, by using Cropwat, the water requirement was estimated at 295.8 mm. The evapotranspiration estimated by the FAO Penman-Monteith equation was 4.8 percent less than the actual amount of water requirement. Therefore, the accuracy of this equation is acceptable for Khorramabad region. The average seed yield was 723 kg per hectare. According to the results of this research, it is recommended that the following information could be used for Khorramabad region: Crop coefficient 0.488, 0.955, 1.083 and 0.371 for initial, development, middle, and ending growth stages, respectively, and water requirement for chickpea obtained equal to 438.04 mm. Using the obtained information for Khorramabad region and similar areas in the country, optimal irrigation management of chickpea will be possible.