干湿交替下基于超声波速度的土壤含水率估算模型

为探究不同墒情需求的农田耕作层土壤含水率与超声波速度的变化关系，采用室外模拟降雨的方式，使土壤样本初始含水率分别为15%、20%、25%和30%，之后置于自然环境下干燥直到土壤含水率达到5%结束干燥，每种处理进行共4次干湿交替，利用超声波土壤含水率原位检测装置对土壤样本进行超声波速度测定。结果表明：土壤样本在各次干湿交替过程中随着土壤含水率的不断下降，土壤容重及超声波速度均呈非线性的增加。随着干湿交替次数的增加，土壤含水率变化对超声波速度的影响减弱，土壤样本初始含水率越高，干湿交替次数对超声波速度的影响越小。采用自适应加权数据融合算法将多次干湿交替过程中的土壤样本超声波速度加权融合，并利用非线性回归分析构建适用于经历多次干湿交替作用下的超声波速度-土壤含水率关系模型，预测误差在6%左右，表明该模型可用于描述不同墒情需求的农田耕作层土壤含水率与超声波速度的关系。研究结果可为利用超声波速度特性实现不同灌溉性质农田土壤含水率的持续监测及预测提供参考。

Model for estimating soil moisture content based on ultrasound velocity under dry-wet alternating conditions

Soil moisture content is an important parameter for the management of farmland irrigation and research of regional hydrological conditions. It is an important prerequisite for scientific irrigation management to acquire the information of soil moisture and understand the dynamic change of soil moisture timely and accurately. However, the existing soil moisture content detection methods have some disadvantages, such as complex analysis process, long cycle, high cost and difficult to be widely used in practical applications. As a good information carrier, ultrasonic wave carries information related to soil moisture content in the receiving wave when it propagates in soil medium and interacts with soil. So, Ultrasonic testing technology is a way, which can monitor soil moisture quickly and in timely. In this study, taking the soil of farmland tillage layers as research specimens, the velocity of ultrasonic pulse through the soil sample from wet to dry was measured by ZBL-U510 non-metallic ultrasonic detector. The relationship between the velocity of ultrasonic wave and the soil moisture content was studied under different soil moisture conditions. The initial moisture contents of soil samples were 15%, 20%, 25% and 30%, respectively, and then dried to 5% in the natural environment. Each treatment sample experienced 4 dry-wet alternations. The results showed that the soil bulk density and ultrasonic velocity increased nonlinearly with the decrease of soil moisture content in the process of soil sample from wet to dry and the change relationship was similar in each wet-dry process. The change of soil moisture content had the greatest influence on the ultrasonic velocity in the first wet-dry process. In order to consider the effect of the dry-wet alternation on the ultrasonic velocity, an adaptive weighted data fusion algorithm was adopted. Firstly, the ultrasonic velocity values corresponding to soil sampling points for moisture content measurement in the processes of dry-wet alternation were fused in cross batch, and four ultrasonic velocity fusion values and mean square deviation were obtained. Then, the weighted factors of each group were calculated according to the multiple extreme value theory. Finally, the final fusion value of ultrasonic velocity was obtained according to the adaptive weighting model and the introduced weighting factor. Through the analysis of the relationship curve between the fusion value of ultrasonic velocity and the soil moisture content, the ultrasonic velocity increased nonlinearly with the continuous decrease of the soil moisture content in four different types of dry and wet alternation process, and the ultrasonic velocity increased rapidly with the decrease of the soil moisture content in the stage of high moisture content. The increasing rate of ultrasonic velocity was slowed down when the moisture content of soil samples ranged from 7%, 9%, 11% to 12%, which was basically consistent with the corresponding moisture content when the ultrasonic pulse velocity slowed down in each wet-dry alternate process. According to the relationship between the fusion values of ultrasonic velocity and moisture content, a polynomial model suitable for different initial moisture content was established. The relative error between the predicted value and the measured value of the model was less than 6%, which showed that the composite polynomial model can be used to accurately describe the relationship between soil moisture content and ultrasonic velocity. The results of this study can provide valuable information for the continuous monitoring and forecasting of soil moisture information in farmland by using the characteristics of ultrasonic velocity.