Development of irrigation control method based on evapotranspiration from the plant canopy

用一个简单的植物冠层模型的热平衡方程，推导出Penman-Monteith方程，用于估算植物冠层的蒸散量。向下和向上的入射辐射、冠层上方的空气温度和风速是仅有的四个需要测量的因素。在干湿条件下，估算的蒸散量与精确尺度测量的值吻合良好。该方法简便，可用于大棚灌溉控制。发现平面传感器的余弦定律会引起误差，因为接收太阳能量的植物冠层表面不是平坦的。为了研制一种经济的太阳辐射计来测量与太阳直接辐射垂直的表面所接收的太阳辐射，在一个不透明的白色乒乓球底部放置了一个光电二极管，在乒乓球底部有一个供光电二极管使用的孔。结果表明，该日光计可用于温室环境控制，形成较好的控制策略。实践证明，该系统在大棚中使用效果良好。在开阔地，应考虑降水的影响。使用四个检测湿度的传感器来确定降雨后何时开始灌溉，并使用电位器来测量土壤含水量。通过判断蒸散发开始的时间，可以由研制的仪器自行确定时间。

A simple heat balance equation for the plant canopy model from which the Penman-Monteith equation was derived is used to estimate evapotranspiration from a plant canopy. Incoming radiation downward and upward, air temperature and wind speed above the canopy are the only four factors to be measured. Estimated evapotranspiration is in good agreement with the values measured by an accurate scale in wet and dry conditions. The present method was simple and considered for irrigation control in greenhouses.It was found that the cosine law for a flat sensor causes an error since the surface of a plant canopy which receives the sun's energy is not flat. In order to develop an economical solarimeter to measure solar radiation received by the surface normal to the direct solar radiation, a photodiode was placed at the bottom of an opaque white ping-pong ball which had a hole for the photodiode at the bottom. It was concluded that this solarimeter can be used to form better control strategies for greenhouse environment control.It was proved that the system can be used without any problem in greenhouses. In the open field, the effect of precipitation should be taken into account. Four sensors to detect wetness have been used to determine when irrigation should be started after rain fall with a potentiometer to measure soil water content. It was concluded that the timing can be determined by the developed instrument itself by judging when evapotranspiration starts.