The regulation of water consumption by mountain jujube trees, jujube pear-like, has not been previously researched in a systematic fashion because of differences among the jujube samples, including age, variety, canopy morphology, meteorological conditions, and land surface conditions. Numerical simulation is an effective and convenient method to investigate the patterns of water consumption of trees, including jujube trees. Therefore, this paper introduces a mathematical model (HYDRUS-1D) to simulate the soil moisture requirements for jujube pear-like.
First, to develop the model, the forecast parameters of the HYDRUS-1D model, such as soil properties, weather conditions, leaf area index (LAI), root distribution, and basal crop coefficient, were measured in the northern hilly area of Shannxi Province, China, from 2008 to 2010. Second, the dynamic characteristics of soil moisture in a rain-proof plot experiment were roughly simulated using the HYDRUS-1D model, then, we adjusted the model so that the simulated results better fit the measured data of soil moisture. We inversely extrapolated the parameters of soil water stress coefficient and extinction coefficient, which determine the values of the potential transpiration and potential evaporation, respectively, for the atmospheric boundary conditions. Finally, using the parameters we estimated, we used the HYDRUS-1D model to predict water consumption by eight-year-old jujube pear-like trees in the field.
The results showed the following. (1) The changes in LAI of jujube pear-like were consistent with a Gaussian function from germination to fruit ripening. Average LAI of jujube pear-like increased from young to old trees, but the inter-annual differences in LAI decreased in trees that were older than six years. (2) Root depth of jujube pear-like increased linearly with the age of the tree. At the same time, the distribution of capillary roots decreased linearly with an increase in the root depth. The percentage of capillary roots at a depth of 40 cm was more than 70%. (3) To obtain the boundary conditions of the HYDRUS-1D model, we used LAI and the extinction coefficient combined with the crop coefficient to calculate the daily reference evapotranspiration. Fitting the model to the data indicated that, when extinction coefficient of mountain jujube pear-like was 0.52, the measured data of soil moisture fit the simulation quite well. (4) The correlation coefficient of simulated values and measured data of soil water storage reached 0.89, which proved that the parameters of the model with respect to water consumption of jujube pear-like were reliable and that the HYDRUS-1D model could simulate well the dynamic process of soil moisture changes and accurately estimate the water requirements of jujube pear-like. (5) As a result, cumulative evaporation of mountain jujube pear-like was 92 mm, and cumulative transpiration was 175 mm. The total water volume consumed by eight-year-old jujube pear-like trees, from germination to harvest, was 267 mm. Based on the actual amounts of local rainfall and water resources, jujube pear-like should be irrigated two to three times during its flowering and fruiting periods, and water application depths should reach 30-40 mm to meet the growth requirements of this crop plant.