(This has prompted biologists to say that “rainforests make their own rain.”)
蒸发（Evaporation） 植株间土壤或田面 蒸发的水分 refers to the movement of water from the soil surface or water surface in paddy field into the atmosphere
蒸腾（Transpiration）
蒸腾（Transpiration）
Transpiration takes place so quickly, that in a rainforest, water that is in the soil in the morning travels into, up, and out of a tree in time to become rain in the afternoon.
第一节 作物需水量
一、基本概念
二、研究作物需水量的意义
三、作物需水量的推求方法
一、基本概念
1、作物需水量 2、田间耗水量
1、作物需水量
（Crop water requirement）
植株蒸腾 Traspiration
作物需水量 = 植株蒸腾 + 株间蒸发=腾发量 Evapotranspiration(ET)
ET E0
ET E0 b
与ET同时段的 水面蒸发量
某时段内的 作物需水量（mm） 国内外都有较多应用，我国尤其在水稻 地区应用较多
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.
evaporation from the soil surface is at most equal but usually considerably less than evaporation from an open water surface
2、田间耗水量
（ Consumptive Use of Water） 旱地耗水量 = 作物需水量 稻田耗水量 = 作物需水量 + 田间渗漏
The evapotranspiration of a crop is the total amount of soil water used for transpiration by the plants and evaporation from the surrounding soil surface. 株间蒸发 Evaporation The evapotranspiration is commonly expressed in millimetres of water used per day (mm/day)

直接计算作物需水量的方法
从影响作物需水量的主要因子（水面蒸发、气温、 湿度、日照、辐射等）中选择单因子或多因子， 通过实测数据的相关分析，建立经验公式
以水面蒸发为参数的需水系数法）
以产量为参数的需水系数法
直接计算作物需水量的方法
以水面蒸发为参数的需水系数法
Pan Evaporation Method
蒸腾（Transpiration）
作物根系从土壤中吸入 体内，通过叶片的气孔 扩散到大气中去的水分 refers to the movement of water from the soil into a plant, up the plant body, and out the plant’s leaves into the atmosphere.

水稻田的渗漏 Percolation in paddy field
Percolation
影响作物需水量的因素
Factors Affecting Crop Water Requirement
Factor Climate Effect on crop evapotranspiration high low hot cool dry wet windy no wind no clouds cloudy mid/late initial or season ripening dense wide plant plant spacing spacing moist dry
二、研究作物需水量的意义

Design new irrigation systems

schedule (operate) existing ones
三、作物需水量的确定
Determining Evapotranspiration – Direct measurement (Lysimeters) – Calculated (equations)
Crop
Soil moisture
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
1、作物需水量的观测
Measurement of Evapotranspiration
Lysimeter
Measuring Crop ET
Inside view of weighing lysimeter
1995 Soybean Crop
3m
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