Estimating Plant Available Water Using Pedotransfer Function Classes to Better Address Water Stress in Agriculture and Forestry in Togo

Plant available water (PAW) also known as total available water (TAW) - the amount of water that the soil can store and return to plants for their physiological needs – is the essential parameter of soil water balance models, which are themselves integrated into crop growth and orientation models (crop type, rotation, estimation of production potential) and in irrigation decision-making tools (irrigation management, water deficit assessment, drainage calculation). Water balance calculation also has multiple applications in forestry: (i) explaining forest dieback phenomena, (ii) defining the suitability of sites for the silviculture of different species by assessing the associated water risk, (iii) defining the canopy density - water resources trade-off for well-considered management of thinning to protect trees from excessive competition for water resources, (iv) developing decision-making tools for forest management. However, soil hydraulic parameters used to calculate the TAW, in particular the water content at field capacity and at permanent wilting point, are often not available from soil maps and their measurement is very time consuming and costly. This study, conducted between April and July 2024, aims to estimate the TAW of Togo soils, using pedotransfer function classes (Class PTFs). The performance and reliability of Class PTFs used were first validated by calculating the percentage of applicability, the mean error and the root mean squared residual error by testing 30 soil samples. The TAW estimating was performed on a set of 79 horizons from 50 reference site profiles used to characterise the different types of soil and draw up the soil map of Togo. Spatialization was carried out using QGIS v 2.18.28 software. The results show that over three-quarters of Togo's territory has a TAW above 250 mm, essentially ferrallitic soils (260 to 521.25 mm), hydromorphic tropical ferruginous soils (318.1 mm) and Mull soils (280.55 to 291.9 mm). This study provides a simple diagnostic mapping tool for farmers and forest managers. It introduces a spatial quantification of soil water availability in Togo, which is important for the country's development, as well as for establishing good practices in irrigation and crop management. Moreover, this type of information can support a diverse range of studies and projects aimed at mitigating climate change.