Advances in research of controlled drainage for crop production and environmental protection

Abstract: Controlled drainage, also known as water table management, is an environmental friendly agricultural water management practice. It has been widely advocated in recent years to reduce drainage discharge in order to reduce agricultural non-point source pollution while ensuring crop production. Further research and application of the controlled drainage are of great importance to drainage system design and water management in a changing environment. In this paper, we reviewed the development history, multiple functions and research advances of the controlled drainage in China and the world. The development of controlled drainage technique has experienced several stages, advancing from its original purpose of water conservation to later goals of nutrient loss reduction, and irrigation water saving etc. Implementation of controlled drainage can be performed easily with water level control structures installed at the outlet of drainage ditch or pipe. However, properly schedule water level adjustment at the outlet remains a challenge, considering the variable weather conditions and different crop drainage requirements. In saline agricultural environment, controlled drainage has to meet the requirement of salinity control. Existing research has demonstrated that controlled drainage can achieve multiple benefits in reducing nutrient losses and conserving water in humid regions, and controlling soil salinity and saving irrigation water in arid and semi-arid regions. Controlled drainage in coastal regions has additional benefits of reducing rain water losses and lowering the risk of salt water intrusion. While controlled drainage research in China is relatively lagged behind, this water table management technique has long been used by grass root farmers to reduce drainage intensity for less irrigation requirement. For many areas in China, where rice and dry foot crop are rotationally cultured, controlled drainage has the advantages of adjusting drainage intensity flexibly to meet moisture level requirement of different crops. The present need in controlled drainage is to make appropriate water level control schedules that are easy to implement and consistent with local crop production requirement. Computer modeling has been widely used to examine long term effect of controlled drainage on hydrology and nitrogen losses. DRAINMOD model is the mostly used simulation tool that predicts outcome of different drainage system layout under variable weather, soil and cropping conditions; the nitrogen module-DRAINMOD_NII enables the model to predict nitrogen losses from drained agricultural fields under different water management, tillage and fertilization practices. Lacking of field observations is the major obstacle in applying modeling approach for controlled drainage research in China. In summary, we conclude that controlled drainage is a necessary practice for modern agricultural drainage; providing benefits in regulating soil moisture in the drained fields, reducing nutrient losses, saving irrigation water use and increasing rainwater use, controlled drainage is the best practice for drainage water management for sustainable agricultural development.