Study On The Wter Balance Of Soil-plant-atmosphere Continiuun Of Farmland In The Gully Region Of Loess Plateau

Soil water balance method, eddy covariance, and microclimate monitor system were employed in this study to investigate water budget at the different spatial and temporal scales. The study site is located in Changwu tableland which is typical representative of the gully region of Loess Plateau. On the basis of fixed-point monitoring, we analyzed the canopy interception of spring corn and its influencing factors under natural rainfall, the dynamics of diurnal evapotranspiration and its influencing factors, the cropland soil water content in different spatial and temporal variations under the cropland-apple orchard land use pattern, and evaluated the water budget status of cropland. The main results were as follows:1. The result showed that under the different rainfall levels 0.1-4.9 mm, 5.0-14.9 mm,15.0-29.9 mm,), the canopy interceptions were 1.1 mm, 2.0 mm, 13.0 mm and the average of canopy interception was 1.7 mm. The canopy interception rates were 12.3%, 12.1%, 15.3% and the average of canopy interception rate was 13.3%. A linear regression model was developed to express the relationship of canopy interception rate with leaf area index and plant height. A power function between canopy interception and vapor pressure deficit and rainfall duration was founded. An exponential function between canopy interception and the rainfall amount was developed. Finally, a regression model was considered to express the relationship between corn canopy interception and possible influencing factors(R2=0.946).2. The result showed the rainfall influenced evapotranspiration significantly. Diurnal evapotranspiration increased after rainfall. The variation coefficient of soil water in the 0-100 cm soil layer was comparatively large, indicating soil water content changed dramatically. Crop root was distributed mainly between 0 and 80 cm, so soil water of 0-100 cm soil layer was mainly involved in evaporation process. The cumulative amount of evapotranspiration in sunny day was greater than that of the cloudy day, while the start time of cloudy evapotranspiration was later than that of sunny day. Meantime the evapotranspiration under cloudy condition was more susceptible to the disturbance of meteorological factors. The net radiation was the main influencing factors of evapotranspiration rate under the different weather conditions, and the evapotranspiration rate was consistent with the trend of net radiation, but the time lag in the net radiation. Under different soil water content conditions, the process and intensity of evapotranspiration was different. Under water stress condition, the evapotranspiration was lower throughout the whole day, and “evapotranspiration plateau” had long duration, while under no-water stress condition, the evapotranspiration was higher, and “evapotranspiration plateau” had short duration with a high evapotranspiration rate for a long time.3 The result showed the soil moisture content in orchards and cropland was affected by precipitation and has consistent variation in real time. The cropland soil water content in each layer(0-20 cm, 20-60 cm, 60-100 cm, 100-200 cm, 200-300 cm) was larger than the orchard, especially in 200-300 cm layer. These was due to the different vegetation evapotranspiration and physiological needs of different water content, which resulted the different land use patterns under the different soil layers of soil water content differences. With the distance increase from orchard to cropland soil water content monitoring point, the cropland soil water content gradually increased with the distance increases from orchard, the loss of soil water storage gradually reduced, the phenomenon of desiccated gradually disappeared. These were due to the different of distribution of apple tree and crop root difference. During the scarcity precipitation or physiological growing season, the apple tree’s hydrotropism made its roots extend to the water which could reduce soil water content in cropland.4. The result showed the precipitation from 2012 to 2014 was less than the average of many years, the anomalous percentage of precipitation was-17.7%,-10%,-5%, respectively. The proportion of monsoon rainfall was higher than the proportion of annual average precipitation during the rainy season, but for the local conditions, the precipitation was not enough to sustain the crop normal growth. Throughout the period of observation, the equipartition terms was large, precipitation could made the soil water content got a temporary supplement, but the water balance was poor.