Biosystems Engineering (2006) 94 (2), 285–295 doi:10.1016/j.biosystemseng.2006.02.018 SW—Soil and Water Assessment of a Water Activity Meter for Rapid Measurements of Soil Water Potential J.J. Cancela; J. Dafonte; E.M. Martı´nez; T.S. Cuesta; X.X. Neira Department of Agroforestry Engineering, University of Santiago de Compostela, Campus Universitario. 27002 Lugo. Spain; e-mail of corresponding author: cancela@lugo.usc.es (Received 26 May 2005; accepted in revised form 23 February 2006; published online 19 April 2006) 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 Cha´ 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 R 2 X0951 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 ðR 2 ¼ 0543Þ 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). r 2006 IAgrE. All rights reserved Published by Elsevier Ltd 1. Introduction Quantitative assessment of soil available water is very relevant to the establishment of water management guidelines for irrigated crops. The current scarcity of water resources, which is connected to the climatic conditions and the growing demand for water for urban and industrial uses, demands the development of models that can be integrated into a decision support system that allows sustainable management of irrigated areas. Soil available water is a characteristic soil variable, necessary in the application of models for irrigation water management (Mannocchi & Mecarelli, 1993; Smith, 1993; Paz et al., 1996; Pereira et al., 2003; A ´ lvarez et al., 2005; Ortega et al., 2004). Such variable directly affects production (Stricˇevic` & C ˇ aki, 1997) and the decisions about the most suitable moment for irrigation (Zairi et al., 2003; Intrigliolo & Castel, 2004). Soil available water is calculated as the difference between soil water content at field capacity and water content at wilting point, obtained from the water retention curve. There are many different methods to estimate water retention curve (Klute, 1986) with significant differences in terms of the technical aspects and the time required. The water retention curve relates the amount of water and the energy associated with the forces that hold the water in the soil. Both terms are essential for the hydraulic characterisation of the medium studied. From among the techniques available to determine the water retention curve, the pressure plate method is a standard technique used in many studies that analyse ARTICLE IN PRESS 1537-5110/$32.00 285 r 2006 IAgrE. All rights reserved Published by Elsevier Ltd