基于表观热惯量的土壤水分监测

Monitoring soil moisture by apparent thermal inertia method

土壤水分含量是监测农业干旱的重要指标,遥感法是大面积监测土壤水分时空特征的主要方法,热惯量法是遥感方法监测土壤水分的主要研究手段之一。本文提出了一个改进的表观热惯量模型计算表观热惯量,并通过地面验证试验对该模型的适用性进行了分析。在中国科学院栾城农业生态系统试验站,通过严格的控制试验,设计了10个不同植被覆盖、不同土壤水分含量的试验小区,针对表观热惯量的适用条件,利用实测的地表温度、植被指数、反照率、太阳辐射等参数计算了不同植被覆盖不同土壤水分含量下的表观热惯量,并与土壤体积含水量进行了相关和回归分析。结果表明:在植被覆盖度较低情况下[归一化植被指数(normalized difference vegetation index,NDVI)<0.35],表观热惯量法具有较好的效果,表观热惯量与土壤体积含水量之间的相关系数大于0.7,通过了95%的显著性检验,两者具有很高的相关性,可以用热惯量法监测土壤水分状况;在较高植被覆盖情况下(NDVI>0.35),表观热惯量与土壤体积含水量之间没有相关性,热惯量法监测土壤水分失效;NDVI为0.35可以作为热惯量法监测土壤水分状况是否可行的判断条件。

Soil moisture is one of the most important indices for agricultural drought monitoring and water resources management.Remote sensing is a critical technology for monitoring spatial and temporal variations in soil water content.The thermal inertia method,which is a thermal infra red（IR） technology,has demonstrated advantages in monitoring soil water condition.Among the several models for computing soil thermal inertia by remote sensing,ascertaining the conditions for monitoring soil water content by thermal inertia remains a major obstacle.This paper proposed an improved model for calculating Apparent Thermal Inertia（ATI）.In the first step,a new soil ATI model with improved algorithms for simulating net radiation was developed.Then a strict control ground experiment was conducted to test the proposed model.A total of 10 experimental plots with different vegetation covers and soil water contents were set up at the Luancheng Agro-Ecosystem Experimental Station of Chinese Academy of Sciences.The vegetation covers were fully representative by NDVI（normalized difference vegetation index）.The actual measured land surface temperature,NDVI,albedo,soil water content,solar radiation and long-wave atmospheric radiation were used to compute ATI under different land cover and soil water conditions.Then correlation and regression analyses were finally done to relate ATI and soil water content.The results indicated that the proposed thermal inertia model reliably monitored the soil water condition,especially in low vegetation cover areas.For low vegetation cover（NDVI 0.35）,the coefficient of determination between ATI and soil volumetric water content was 0.7.The proposed thermal inertia method was invalid for NDVI 0.35 and the corresponding coefficient of determination was 0.2.NDVI that was the equivalent of 0.35 could be critical for determining the applicability of the proposed model in monitoring soil water conditions.This was because temperature dynamics（the most critical criteria for calculating ATI） for bare and vegetated lands were different.However,the proposed model was not only simple,but it carries distinct physical meaning and easy-to-use interfaces.The experiment suggested that the model was applicable in reliably monitoring soil water conditions.