第二章 作物需水量与灌溉用水量.ppt

气象因素（Climatic factors） Radiation: supplies energy to evaporate water from liquid to vapor Air temperature: determines how much water air can hold Humidity: how much water is in the air Wind: how easily a molecule moves into the atmosphere Design new irrigation systems Direct measurement (Lysimeters) Measuring Crop ET 2、作物需水量的计算Calculation of ET 直接计算需水量 通过计算参照作物需水量来计算实际作物需水量 Etp = Epan * Pan factor (0.8) * Crop factor Epan = Class A pan evaporation. This data is interpolated for each location based on data from major regional centres. Pan factor = Evaporation of water from a pan is affected by the pan’s dimensions, wind speed, humidity and the type of crop within the upwind fetch. The pan factor is typically 0.8 but may range from 0.45 to 0.85. Crop Factor = Empirical ratios of crop to pan evaporation. Its value is indicative of the green leaf area of the vegetation. Energy balance equations Rn= net radiation [MJ m-2 d-1] Rns= net shortwave radiation [MJ m-2 d-1] Rnl= net longwave radiation [MJ m-2 d-1] Ra= extraterrestrial radiation [MJ m-2 d-1] where: Ra= extraterrestrial radiation [ML m-2 d-1] dr= relative distance Earth - Sun dr = solar declination [rad] ψ= latitude [rad] ωs= sunset hour angle [rad] The soil heat flux (G) is estimated for a daily periods (effective soil depth 0.18m ) as follows: For monthly periods (effective soil depth 2.0 m) the soil heat flux (G) can be estimated as: Since the magnitude of daily soil heat flux over 10-30 day periods is relatively small, it normally can be neglected and thus G = 0 Slope Vapour Pressure Curve (D) D= slope vapour pressure curve [kPa oC-1] T= air temperature [oC] ea = saturation vapour pressure at temperature T [kPa] Psychrometric Constant γ= psychrometric constant [kPa oC-1] P= atmospheric pressure [kPa] l= latent heat [MJ kg-1] Atmospheric Pressure Assuming atmospheric pressure at sea level Po= 101.3 [kPa]: z = elevation above sea level [m] b. I