Leveraging Biochar to Reduce the Water Footprint in Agriculture: A Comprehensive Review

Biochar (BC) has emerged as a soil amendment for improving water retention, enhancing crop productivity, and reducing the agricultural water footprint, particularly in water-limited environments. This review synthesizes current scientific evidence on the role of BC in modifying soil physical, chemical, and hydrological properties that directly influence green, blue, and gray water footprints. BC can modify soil porosity, water-holding capacity (WHC), and field capacity (FC), while influencing bulk density, evaporation, runoff, and deep percolation. These effects can improve plant-available water (PAW) and enhance irrigation water use efficiency (IWUE), potentially reducing the blue water footprint. Improved infiltration, reduced runoff, and enhanced soil aggregation can improve the utilization of effective rainfall and potentially reduce green water footprint. BC can also limit nutrient and contaminant leaching through adsorption and microbial immobilization, potentially reducing the gray water footprint. Synergistic interactions between BC and mulching, micro-irrigation, and conservation agriculture further enhance IWUE and crop water productivity. Crop-modeling tools such as AquaCrop, Soil & Water Assessment Tool (SWAT), and Decision Support System for Agrotechnology Transfer (DSSAT) help assess potential BC effects on water productivity and simulate its impacts at both field and watershed scales. Key research gaps include limited availability of long-term field studies, lack of standardized water footprint assessment methods, and insufficient understanding of BC interactions with climatic variability and management practices. Overall, BC shows substantial potential to support sustainable water management and climate-resilient agriculture.