Assessment of a Water Activity Meter for Rapid Measurements of Soil Water Potential

Permanent wilting point (PWP) is required to calculate soil available water, a parameter that must be known for good irrigation water management. This work assesses the use of a water activity meter to determine the PWP in the Terra Chá irrigation district, located in Galicia, northwest Spain. In addition, the study compares this assessment with the results obtained using a pressure plate. Twenty-four samples of disturbed soil were collected at a depth of 0–20 cm. The samples corresponded to different types of soils and crops. This study found good repeatability of the method used for PWP estimation. Two methodologies were tested to develop the soil water retention curve. The initial methodology estimated the water retention curve based on eight water contents and four readings per water content, while the alternative methodology estimated the curve based on four water contents and two readings per water content. The values obtained for PWP using the two methodologies were similar (coefficient of determination image for 1:1 line, in all cases). The most appropriate methodology was the alternative methodology because the time required to perform the process was shorter. The PWP values estimated with pressure plate were compared with the values estimated using water activity meter according to the alternative methodology. An average correlation image was obtained. The readings taken with the water activity meter showed lower values for PWP estimation. The time required to estimate PWP decreased when the water activity meter was used according to the alternative methodology (4·5 h), as compared to the time required when the pressure plate was used (9 h).