Abstract：Soil water extracted by tree roots can cause significant decrease of the soil water content and eventually lead to ground settlement. Understanding water uptake by tree roots will help to know the influence of tree on the ground settlement. The main objective of this paper is to develop a flexible and applicable root water uptake model for trees. For this purpose，a serious of measurements and tests were firstly conducted in an experimental citrus orchard，including experimental studies on water kinematical parameters of the unsaturated soil，monitoring of the soil water potential，survey of citrus roots and observation of meteorological elements. And then an inverse calculation method of soil water movement was demonstrated. With this method，root soil water uptake rates can be obtained when the soil water potentials are available. Based on the measured data，a root water uptake model was developed using multiple regression analysis method. The root water uptake pattern calculated using this root water uptake model is approximate to the measured root water uptake pattern. Compared with the existing root water uptake models，the model has some advantages in following aspects：firstly，it contains only three easily determined influencing factors，i.e. evaporation of water from water surface，root density function and soil water potential，which make its application very convenient. Secondly，unlike all the existing microscopic root water uptake models that assume root water uptake rate being proportional to root length density，this paper defines root density as the mass of effective dry roots within a unit soil bulk volume because water uptake rate of activity root depends on both the root length and the root radius，and also considers that root water uptake rate is an exponential function of root density because the adjacent roots are limited each other to extract soil water. Finally，in this model，four undetermined parameters account for the point of maximum root density and the point of zero root density，which make the root water uptake model universally be suitable for all plants and allow spatial variations of water uptake rate. The four parameters can be easily determined through root density measurement.
