Comment on hess-2022-166

Abstract

Multiyear drought has been proved to cause non-stationary rainfall-runoff relationship. But whether this change can occur in catchments that have also experienced vegetation change and whether it invalidates the most widely used methods (the paired-catchment method (PCM), the time-trend method (TTM), and the sensitivity-based method (SBM)) for estimating impacts of vegetation change on runoff is still unknown and rarely discussed. In the Red Hill paired experimental catchments in Australia, which has experienced a 10-year drought (2000–2009) and afforestation, estimated inconsistent afforestation impacts were 32.8 %, 93.5 %, and 76.1 % of total runoff changes by the PCM, TTM and SBM, respectively. In addition to afforestation, multiyear drought has also led to the non-stationary rainfall-runoff relationship of the paired catchments. For the TTM and SBM, traditional application did not further differentiate different drivers of non-stationary rainfall-runoff relationship, which led to significant overestimation of afforestation effects. A new framework was proposed to separate the effects of three factors on runoff changes including vegetation change, climate variability and multiyear drought caused non-stationarity. Based on the new framework, impacts of afforestation on runoff were 38.8 % by the TTM and 21.4 % by the SBM, agreeing well with that by the PCM (32.8 %). Using paired-catchment observations, this study proved multiyear drought can induce non-stationary rainfall-runoff relationship and proposed a new framework to better separate the impact of vegetation changes on runoff under climate-induced non-stationary condition. More importantly, paired-catchment method is proven to be still the most reliable method even the control catchment experienced climate-induced shift in rainfall-runoff relationship.