Soil moisture routing modeling of targeted biochar amendment in undulating topographies: an analysis of biochar's effects on streamflow

The effect of biochar on hydrologic fluxes was estimated using a single hillslope version of a gridded soil moisture routing (SMR) model. Five grid cells were aligned linearly with varied slopes to simulate a small undulating hillslope with or without a restrictive layer beneath the soil profile. Biochar amendments (redwood sawdust and wheat straw biochar) at concentrations of 0%, 4%, and 7% were applied to the topmost grid-cell by mass of dry soil. Simulated streamflow hydrographs for restricted and non-restricted soil profiles were manually calibrated with measured Palouse River streamflow data. Evapotranspiration, percolation, lateral flow, baseflow, and streamflow were all modeled yearly. Two generally reported field capacities (FC) in literature at −6 and −33 kPa were considered to assess the effect of biochar. Field capacity considered at −6 kPa corresponds to higher moisture content, and hence higher moisture storage capacity between FC and permanent wilting point than at −33 kPa. At −6 kPa FC, biochar effectively increased evapotranspiration and reduced the lateral flow of the system. Increased soil porosity from biochar amendment enhanced the water holding capacity of the soil and plant available water. These mechanisms impacted the streamflow generated from the system indicating positive outcomes from biochar amendment in both restricted and non-restricted soil profiles. Biochar amendment showed an order of magnitude smaller effects with −33 kPa FC compared to −6 kPa FC; the increased porosity appeared to be less influential at lower field capacity values. Additionally, the results showed that the over-application of coarse biochar might negatively affect retaining soil moisture. These findings point to positive results for using biochar as a water management strategy if applied less than 7% in this study, but further exploration is needed to find the optimum level of biochar with different biochar and soil properties.