Development Of Mechanistic Model For Water Dynamics In The Soil-crop System

The water resources per capita in China are insufficient,and the water shortage in agriculture is still severe.The status and changes of soil moisture determine the capacity of crops' absorption of soil water.Root water uptake has a major impact on soil moisture dynamics.Root water uptake models have become a mathematical tool for quantifying root water absorption.Soil moisture can now be dynamically simulated,and the changes in soil moisture under crop evapotranspiration can be predicted.It is,therefore,of great significance to employ numerical models for efficient use of water resources,increase in crop yield and saving water use in agriculture.Due to poor mechanism in the existing macroscopic root water uptake models,this study focused on the development of the mechanistic model.The factors that control root water uptake were changed from simple soil water potential to leaf water potential,soil water potential and root hydraulic conductivity.The processes water movement from soil to root surface and from the root surface to leaf were mechanistically simulated.A root water uptake model was proposed based on root hydraulic conductivity,root length density distribution,leaf water potential,root surface water potential and root radius.Based on the SWMS-2D software,the water uptake module was newly developed,and a computer program was developed to quantify the effect of different root hydraulic conductivity on water movement in the soil-plant system.The following preliminary conclusions were obtained from the numerical simulations of two sets of root hydraulic conductivity and two transpiration(continuous and interval)methods.Under the continuous evaporation condition,when the potential daily evapotranspiration and the root conductivity were both small,crop reached the point of transpiration suppression earlier,but the total amount of root water uptake was large.The distribution soil water potential was more uniform,which benefited use of soil water resources more fully.During the interval transpiration,the point of transpiration suppression occurred even earlier,and the total amount of root water uptake was less than that under the continuous transpiration condition.The effect of root hydraulic conductivity on root water uptake is regulated by both the magnitude and the form of potential transpiration.When the daily potential transpiration is small,the continuous transpiration is more advantageous for root systems to make more full use of soil moisture.Under the interval transpiration condition,the difference in water potential between root surface and the leaf decreased with decreasing in soil water potential.The dominant factor controlling root water uptake is in the process of water movement from the root surface to the leaf,given soil water supply is sufficient.The more uniform the root distribution,the greater the amount of total water uptake,if the other conditions remain identical.