The impact of phosphorus on projected Sub-Saharan Africa food security futures

Sub-Saharan Africa must urgently improve food security. Phosphorus availability is one of the major barriers to this due to low historical agricultural use. Shared socioeconomic pathways (SSPs) indicate that only a sustainable (SSP1) or a fossil fuelled future (SSP5) can improve food security (in terms of price, availability, and risk of hunger) whilst nationalistic (SSP3) and unequal (SSP4) pathways worsen food security. Furthermore, sustainable SSP1 requires limited cropland expansion and low phosphorus use whilst the nationalistic SSP3 is as environmentally damaging as the fossil fuelled pathway. The middle of the road future (SSP2) maintains today’s inadequate food security levels only by using approximately 440 million tonnes of phosphate rock. Whilst this is within the current global reserve estimates the market price alone for a commonly used fertiliser (DAP) would cost US$ 130 ± 25 billion for agriculture over the period 2020 to 2050 and the farmgate price could be two to five times higher due to additional costs (e.g. transport, taxation etc.). Thus, to improve food security, economic growth within a sustainability context (SSP1) and the avoidance of nationalist ideology (SSP3) should be prioritised

Soil chemistry aspects of predicting future phosphorus requirements in Sub-Saharan Africa

Phosphorus (P) is a finite resource and critical to plant growth and therefore food security. Regional‐ and continental‐scale studies propose how much P would be required to feed the world by 2050. These indicate that sub‐Saharan Africa soils have the highest soil P deficit globally. However, the spatial heterogeneity of the P deficit caused by heterogeneous soil chemistry in the continental scale has never been addressed. We provide a combination of a broadly adopted P‐sorption model that is integrated into a highly influential, large‐scale soil phosphorus cycling model. As a result, we show significant differences between the model outputs in both the soil‐P concentrations and total P required to produce future crops for the same predicted scenarios. These results indicate the importance of soil chemistry for soil‐nutrient modelling and highlight that previous influential studies may have overestimated P required. This is particularly the case in Somalia where conventional modelling predicts twice as much P required to 2050 as our new proposed model. Plain language summary Improving food security in Sub‐Saharan Africa over the coming decades requires a dramatic increase in agricultural yields. Global yield increase has been driven by, amongst other factors, the widespread use of fertilisers including phosphorus. The use of fertilisers in Sub‐Saharan Africa is often prohibitively expensive and thus the most efficient use of phosphorus should be targeted. Soil chemistry largely controls phosphorus efficiency in agriculture, for example iron and aluminium which exist naturally in soil reduce the availability of phosphate to plants. Yet soil chemistry has not been included in several influential large‐scale modelling studies which estimate phosphorus requirements in Sub‐Saharan Africa to 2050. In this study we show that predictions of phosphorus requirement to feed the population of Sub‐Saharan Africa to 2050 can significantly change if soil chemistry is included (e.g. Somalia with up to 50% difference). Our findings are a new step towards making predictive decision‐making tool for phosphorus fertiliser management in Sub‐Saharan Africa considering the variability of soil chemistry

Data from: Feedbacks between shallow groundwater dynamics and surface topography on runoff generation in flat fields

In winter, saturation excess (SE) ponding is observed regularly in temperate lowland regions. Surface runoff dynamics are controlled by small topographical features that are unaccounted for in hydrological models. To better understand storage and routing effects of small scale topography and their interaction with shallow groundwater under SE conditions, we developed a model of reduced complexity to investigate SE runoff generation, emphasizing feedbacks between shallow groundwater dynamics and mesotopography. The dynamic specific yield affected unsaturated zone water storage, causing rapid switches between negative and positive head and a flatter groundwater mound than predicted by analytical agro-hydrological models. Accordingly, saturated areas were larger and local groundwater fluxes smaller than predicted, leading to surface runoff generation. Mesotopographic features routed water over larger distances, providing a feedback mechanism that amplified changes to the shape of the groundwater mound. This in turn enhanced runoff generation, but whether it also resulted in runoff events depended on the geometry and location of the depressions. Whereas conditions favourable to runoff generation may abound during winter, these feedbacks profoundly reduce the predictability of SE runoff: statistically identical rainfall series may result in completely different runoff generation. The model results indicate that waterlogged areas in any given rainfall event are larger than those predicted by current analytical groundwater models used for drainage design. This change in the groundwater mound extent has implications for crop growth and damage assessments

Chloride circulation in a lowland catchment and the formulation of transport by travel time distributions

[1] Travel times are fundamental catchment descriptors that blend key information about storage, geochemistry, flow pathways and sources of water into a coherent mathematical framework. Here we analyze travel time distributions (TTDs) (and related attributes) estimated on the basis of the extensive hydrochemical information available for the Hupsel Brook lowland catchment in the Netherlands. The relevance of the work is perceived to lie in the general importance of characterizing nonstationary TTDs to capture catchment transport properties, here chloride flux concentrations at the basin outlet. The relative roles of evapotranspiration, water storage dynamics, hydrologic pathways and mass sources/sinks are discussed. Different hydrochemical models are tested and ranked, providing compelling examples of the improved process understanding achieved through coupled calibration of flow and transport processes. The ability of the model to reproduce measured flux concentrations is shown to lie mostly in the description of nonstationarities of TTDs at multiple time scales, including short-term fluctuations induced by soil moisture dynamics in the root zone and long-term seasonal dynamics. Our results prove reliable and suggest, for instance, that drastically reducing fertilization loads for one or more years would not result in significant permanent decreases in average solute concentrations in the Hupsel runoff because of the long memory shown by the system. Through comparison of field and theoretical evidence, our results highlight, unambiguously, the basic transport mechanisms operating in the catchment at hand, with a view to general applications

Chloride circulation in a lowland catchment and the formulation of transport by travel time distributions

Travel times are fundamental catchment descriptors that blend key information about storage, geochemistry, flow pathways and sources of water into a coherent mathematical framework. Here we analyze travel time distributions (TTDs) (and related attributes) estimated on the basis of the extensive hydrochemical information available for the Hupsel Brook lowland catchment in the Netherlands. The relevance of the work is perceived to lie in the general importance of characterizing nonstationary TTDs to capture catchment transport properties, here chloride flux concentrations at the basin outlet. The relative roles of evapotranspiration, water storage dynamics, hydrologic pathways and mass sources/sinks are discussed. Different hydrochemical models are tested and ranked, providing compelling examples of the improved process understanding achieved through coupled calibration of flow and transport processes. The ability of the model to reproduce measured flux concentrations is shown to lie mostly in the description of nonstationarities of TTDs at multiple time scales, including short-term fluctuations induced by soil moisture dynamics in the root zone and long-term seasonal dynamics. Our results prove reliable and suggest, for instance, that drastically reducing fertilization loads for one or more years would not result in significant permanent decreases in average solute concentrations in the Hupsel runoff because of the long memory shown by the system. Through comparison of field and theoretical evidence, our results highlight, unambiguously, the basic transport mechanisms operating in the catchment at hand, with a view to general applications

FASTR model validation data

File contains water balance results generated with the FASTR hydrological model for three numerical validation experiments

FASTR experiment 1: role of the unsaturated zone moisture profile

File contains water balance results simulated with the FASTR hydrological model for 15,000 combinations of initial groundwater level, soil hydraulic parameters, saturated hydraulic conductivity, and precipitation structure