Abstract:
Continuous technological innovation of farmland irrigation is a critical component of the strategy for dealing with the shortage of water resources and arable land resources around the world, and is a key to ensuring food security for a growing population especially in developing countries such as China. In this study, a Concept of Crop Initiate Drawing Water (CIDW) was proposed, and four basic preconditions for its included: 1) soil is always water unsaturated, and planted crops must be upland crops; 2) external water potential outside the soil-root system is maintained less than atmospheric pressure; 3) the distance between external water and crop roots is kept as short as possible; and 4) a high efficiency water permeable interface capable of enduring the matric potential difference and another form opposite potential difference. For CIDW, there is invariably matric potential difference between the external water and soil-root system, and another mainly opposite potential difference which determinates the operation mechanism and its category. According to the literature, three type of CIDW were identified, namely Pressure Potential Difference (P-CIDW), Osmotic Potential Difference (O-CIDW) and Gravitational Potential Difference (G-CIDW). The characteristics and operating conditions of the these three types of potential differences were analyzed and described. At present, the overwhelming majority of published studies of CIDW have focused on P-CIDW, only few studies have explored O-CIDW and G-CIDW. The significant advances in hardware included: 1) the form of negative pressure in the P-CIDW system has gradually changed from water pressure to air pressure; 2) the devices used to maintain negative pressure are now small, light, require no energy and are high precise(volume and weight are only one to ten percent of that of traditional devices); and 3) a new organic negative pressure water seepage material, namely Polyvinyl Formal (PVFM), has been invented and verified to be significantly superior to traditional materials like ceramics. At the same time, a large body of literature investigating the most efficient and suitable way to apply P-CIDW fully showed: 1) compared with traditional irrigation technologies such as flood irrigation, furrow irrigation and drip irrigation, P-CIDW with suitable parameters could significantly improve crop yield or water use efficiency, nutrient uptake, fertilizer utilization efficiency, enzyme activity, microbial diversity in the soil rhizosphere, and a coupling effect of water and fertilizer; 2) P-CIDW could significantly affect crop physiological and biochemical indexes and soil nutrient availability and spatial distribution, and whether those effects were beneficial or not depended on different research conditions; 3) soil texture and its profile configuration significantly affected soil water and salt transport and distribution under P-CIDW; and 4) by integrating initial soil moisture content and matrix potential, hydraulic conductivity of emitter, head of water supply and time, the cumulative infiltration under P-CIDW could be semi-theoretically and semi-empirically estimated. Finally, the future of CIDW was speculated on, and it concluded: 1) there was an urgent need to develop a basic theory explaining the soil moisture to crop relationship, 2) development of high efficient potential difference interface materials for CIDW was key; 3) the technical standards for P-CIDW should be formulated as soon as possible; and 4) the revolutionary innovation for methods of maintaining pressure in P-CIDW system was needed.