Mapping and validating the root zone plant-available water holding capacity in central Ethiopia

Abstract

Reliable estimates of the root zone plant-available water holding capacity (RZ-PAWHC) are essential for assessing crop water availability and supporting climate-resilient agricultural planning. RZ-PAWHC is calculated by summing the plant-available water holding capacity (PAWHC), adjusted for the soil fine earth fraction (SFEF), over the root zone depth (RZD). Existing Sub-Saharan Africa (SSA) maps of RZ-PAWHC rely on coarse datasets and lack validation. We generated 100 m-resolution RZ-PAWHC maps for maize and wheat across central Ethiopia by integrating national soil datasets, digital soil mapping, pedotransfer function (PTF), and rule-based rootability indices. Thirteen primary soil properties were mapped at 5 cm-thick intervals from 0 to 150 cm depth using Random Forest models. Two additional primary soil property maps—depth to bedrock and drainage class, each providing a single value per pixel—were obtained from Africa-SoilGrids. These properties served as inputs for PTF to estimate volumetric moisture content at field capacity (VMC-FC) and at permanent wilting point (VMC-PWP), and for rootability rules to derive RZD. A coarse fragment map was also used to derive SFEF. RZD was defined as the shallowest of the depth to bedrock, to oxygen shortage (derived from drainage class), to a restrictive soil layer, or to a crop-specific maximum rooting depth. Prediction accuracy of the primary soil property maps ranged from a model efficiency coefficient of 0.17 for coarse fragments to 0.79 for pH-H₂O. Derived maps showed that PAWHC over the 0150 cm depth ranged from 1 to 237 mm (mean = 122 mm), SFEF averaged 89%, reducing total PAWHC by 11%, and mean maize RZD was 124 cm, primarily constrained by bedrock (covering 45% of the mapped area) and oxygen shortage (22%). RZ-PAWHC maps had mean values of 91 mm for maize and 83 mm for wheat, with the lowest values observed in poorly drained, clay-rich soils. Compared with SSA maps for maize, the generated maps indicated higher RZ-PAWHC due to higher PAWHC, higher SFEF (89 vs 83%) and deeper rooting (124 vs 107 cm), with only minimal chemical root restrictions (salinity, sodicity, toxicity; 1 vs 17%). Validation against independent soil and root observations from 50 soil pits (up to 2 m depth) showed that the generated maps outperformed SSA for VMC-FC and VMC-PWP, while the SSA map better estimated PAWHC and SFEF. RZD and RZ-PAWHC were overestimated in the generated maps, whereas SSA underestimated them. Deriving PAWHC, SFEF, RZD, and RZ-PAWHC from measured- instead of from mapped soil properties revealed that errors were mainly driven by inaccuracies in the mapped soil properties rather than by the PTF or RZD derivation rules. These findings highlight the importance of improving the accuracy of critical soil property maps —particularly drainage and depth to bedrock— which most strongly constrained RZD.

 

Keywords: root zone depth, plant-available water holding capacity, pedotransfer function, validation, central Ethiopia