Applicability of soil moisture sensors for monitoring water dynamics in rock: A field test in weathered limestone

Rock moisture in the vadose zone, while recognized as important, is rarely monitored-in part because adequate instrumentation and installation techniques are lacking. The objectives of this work were (a) to test the applicability of a commercially available capacitance soil moisture sensor (EC-5, METER Group) for continuously monitoring the water content of weathered limestone; and (b) to contrast the water dynamics of rock matrix with that of rock fractures. At a site in the Edwards Plateau, Texas, we developed a protocol for installing sensors in limestone pits and tested its effectiveness in reducing erroneous readings caused by installation-induced preferential flow along pit walls. Our results show that rock moisture can be accurately measured with EC-5 sensors, provided some steps are taken to ensure the quality of the installation. These include protecting the sensors by installing them in horizontal shafts rather than on the exposed pit wall, and use of a sealant. Data gathered over 7 mo after installation showed that rock moisture increased significantly only after a 95-mm rainfall event. Because of preferential flow, fractures reached peak water content within 2-3 h after peak rainfall-a response that was one to two orders of magnitude faster than in rock matrix. Limestone stored 40-70% of the water after the 95-mm event, and its water content decreased significantly during summer months. Overall, our results attest to the importance of continuously measuring rock moisture in regions with shallow soils, where it serves as a critical reservoir for vegetation.