The use of geophysical methods to assess and compare soil moisture content under irrigation conditions

Abstract

Drought and water scarcity are common and recurring phenomena in South Africa, and for this reason water management (and more specifically the management of soil moisture content) plays a pivotal role in the success of agricultural activities. The aims of this study was, firstly to compare datasets between the different geophysical methods used (electromagnetic, neutron scattering and frequency domain reflectometry). Secondly, this study aimed at locating and discussing areas of concern located on the pivot under investigation, and lastly, to assess the cost- and labour efficiency between the methods used. In this study two CMD Electromagnetic Conductivity Meters (collectively referred to as CMD, or individually referred to as the CMD Mini-Explorer and CMD Explorer), the CPN 501DR Depthprobe (Neutron probe) and the Diviner 2000 (Diviner) were used to determine the statistical probability of yielding similar and reliable results. Both statistical (T-test and ANOVA analyses) and visual comparisons were made to determine the reliability of each method, as well as to established the most cost- and labour effective method of geophysical analysis of soil moisture content. Gravimetric and volumetric soil moisture content samples were taken and analysed to compare with the three different geophysical methods. During this study, three areas were identified as problematic, which could lead to lower quality and quantity crop production in the specific study area. Furthermore, it was found that the comparison of the volumetric-, Neutron probe- and Diviner datasets had high statistical probabilities of having similar and reliable results, should they have been used separately for measuring SMC. When the CMD datasets were included in the statistical analyses, the results suggested that the CMD would not have similar and reliable results compared to the other methods. This was attributed to the different outputs (volumetric water content as compared to electrical conductivity) given by the different apparatuses. The visual comparisons, on the other hand, showed that there were positive spatial trends between the different methods used. Suggesting that the CMD could indeed be used to produce a reliable indication of SMC measurements (if the proper environmental factors and variables are considered). This study concluded that, when considering a geophysical method for large-scale and long-term use, the CMD is by far the most appropriate method for indicating soil moisture content (considering the high resolution and the labour efficiency of the apparatus). When a geophysical apparatus and method is being considered for small-scale, continuous and long-term soil moisture content monitoring, the Diviner 2000 was the apparatus of choice looking at cost- and labour efficiency.