Variations of sap flow rate and its farmland scale improving method for spring maize in northeast China

Abstract: Maize is one of the main food crop in the northeast China. It is of great significance to understand the sap flow rate variation and its environmental impact factors for optimizing the structure of maize production and achieving high water use efficiency in the northeast China. In order to reveal the daily variation of sap flow rate and its environmental impact factors, to clarify the scaling conversion factors for upscaling individual plant sap flow estimation to the farmland level, and to investigate the characteristics of evapotranspiration (ET) during the filling stage, field experiments were conducted in two consecutive years (2017 and 2018) in the northeast China. The sap flow rates, crop growth indices, meteorological data, soil evaporation and soil water content of the root zone in black soil were determined. The results showed that there was an obvious day-night fluctuation of spring maize sap flow rate, and the average peak values of sap flow rate in the two years were 58.72 and 48.26 g/h, respectively. Rainfall events had a strong inhibitory effect on the sap flow of spring maize, and the sap flow rate increased significantly after rainfall, even higher than that before rainfall. The sap flow rate was quite different among the sunny, cloudy and rainy days. The sap flow rate revealed a multi-peak curve in the cloudy and rainy days. The maize sap flow rate was the highest in the the sunny day, followed by cloudy and rainy days, with the average values of 16.14, 8.28, 3.13 g/h respectively in 2018. The variation of sap flow rate could be regarded as the result induced by the comprehensive effects of various environmental factors. According to the correlation analysis, the sap flow rate had a significant positive correlation with air temperature, wind speed, water vapor pressure and photosynthetic active radiation, and a significant negative correlation with relative humidity and soil temperature. The absolute values of the correlation coefficients between the sap flow rate and air temperature, photosynthetic active radiation, and relative humidity were above 0.8, indicating that they were the main environmental factors affecting spring maize sap flow rate in the black soil region of northeast China. Using the stem diameter, stem cross-sectional area and leaf area as the scaling conversion factors, the individual plant sap flow rates were upscaled to the farmland level during the spring maize filling stage, which were 118.60, 125.07 and 112.86 mm in 2017, and 104.91, 112.96 and 97.68 mm in 2018, respectively. Leaf area index, wind speed, average soil moisture content at 1-m soil layer, and net radiation were selected to establish the soil evaporation forecast model based on the Support Vector Regression (SVR). Then the SVR model was used to estimate the evaporation values that were absent in micro-lysimeter monitoring mainly in rainfall days. The results showed that the Root Mean Squared Error (RMSE) and Coefficient of Determination (R2) of the soil evaporation forecast model based on SVR were 0.39 mm and 0.82. Therefore, the SVR model was reliable to be used for the evaporation forecast. The evaporation values during the spring maize filling stages in the two years were 42.32 and 32.98 mm, respectively. The sum of transpiration (obtained by the three upscaling methods) and evaporation (obtained as mentioned above) in the two years were compared with the ET calculated by water balance method, and the relative errors were all within 20%. The average daily ET values were within the range of 4.22~4.78 and 3.91~4.56 mm/d, respectively during the spring maize filling stages of the two years. The ET calculated by using leaf area as the scaling conversion factor was the closest to the result of water balance method, with an relative error of about 5%. It demonstrated that the leaf area was suitable to be used as the converting factor for upscaling the single plant sap flow to farmland level of the maize field in black soil of northeast China.