Review of multi-scale evapotranspiration estimation and spatio-temporal scale expansion

Abstract: Information of evapotranspiration (ET) at different spatial-temporal scales over croplands is the basis of high-efficient water consumption control and water-saving irrigation management. However, evapotranspiration theory or model parameter at certain scale is highly relied upon the scale. Due to the lack of knowledge of the multi-scale ET spatio-temporal coupling, an incomprehensive understanding to the agricultural water evaluation may lead to incorrect irrigation regime. Therefore, researches on conversion methods for ETs at various spatio-temporal scales are of significant importance for improving regional irrigation regime and maintain sustainable development of agricultural water resources. This study systematically reviewed the measurement and estimation methods for ETs at various spatio-temporal scales, and evaluated the applicability and pros/cons of each method. A further review of the conversion method for ETs at various spatio-temporal scales was given, presenting future study highlights, including the current research focuses such as cooperation & coupling response mechanism of crops for multi-factors, the integration technology of multi-source ET data from various methods at different spatio-temporal scales, the theoretic conversion model at various spatio-temporal scales, and spatio-temporal coupling scales and water-heat-carbon in a unified system. And the future research emphasises were detailed shown as follow:1) Three aspects of research highlights on ET estimation method were listed below. Firstly, the research highlights on ET estimation model mainly focus on integrating the response mechanism of the crops to many factors such as water, fertilizer, air, heat, light, soil and living beings, building up a regulation & control equation for quantitative coupling and developing the ET estimation method of heterogeneous underlying surfaces in several combination modes including partial irrigation and film covering. On the other hand, the key research on ET measuring method comprises the methods for improving the performance and optimizing the layout of the sensors as well as promoting the ET precision of remote-sensing monitoring area, including the higher parameter precision of the characteristics of the underlying surface in remote-sensing inversion, inversion error analysis and quantitative attributes. Furthermore, the fusion technology of multi-source ET monitoring data at several methods and multi-time/space dimensions, and real-time water shortage diagnosis and precisely controlled irrigation for the crops based on ET information.2) For conversion methods for time-space dimensions of ET, the following research highlights needs to be paid more attention. Building up the theoretical formula for ET conversion between the dimensions with physical meaning including leaf, single plant, field block, farmland and region (irrigation area) are the key research focuses on conversions among space dimensions of ET. While, for conversion of time dimension of ET, the key researches mainly aims at quantizing the error and uncertainty of the conversion of time dimension of ET in different climates, different crop types, different irrigation systems and different managing modes, and optimizing and recommending the conversion function of time dimension for different regions. With regard to the coupled conversion of time and space dimensions of ET, the development of coupled conversion method for time and space dimensions of ET with physical meaning is the pivot in future. The last but not least, with regard to the expansion and application of the conversion of time and space dimensions for ET, the key research need focus on the evaluation of water utilization efficiency in agriculture at different time and space dimensions as well as the coordinated multi-dimension improvement mode for high-efficient water utilization in agriculture. Over all, the integration of water, heat and carbon in one unified system and the development of ET conversion systems at corresponding time and space dimensions will become the hot spot in this field.