Baseflow adjusted snowmelt runoff and rainfall runoff from TerraClimate

We update the total runoff and runoff partitioned from snowmelt from TerraClimate using a modified water balance model that better accounts for baseflow lags in hydrology for use in basin level water supply availability at monthly time scales. These data complement existing TerraClimate 1-d water balance similations, but show much has improved montly correlative skill (median r=0.88) to observed streamflow across watersheds globally watersheds compared with the original method from TerraClimate (median r=0.75). These data can be used to quantify gaps in monthly water demand for downstream water uses including agriculture.

Methods
The original TerraClimate (Abatzoglou et al., 2018) runoff data (q) assumed that all water (rain plus snowmelt, P) in excess of the additional soil water holding capacity (maximum soil water storage minus soil moisture or SR) and monthly evapotranspiration (ET) – or water surplus – immediately runs off that month. 

S = max(0, 0.95*P - ET-SR)

q = 0.05*P + S

While this water balance approach may be valid at local scales, the approach does not account for a variety of factors including baseflow contributions or stream routing and transit time. Some studies show that the transit time for runoff to reach a basin terminus is up to a month (Allen et al., 2018). We updated the approach for simulating runoff to account for the contributions of baseflow and transit time following Wolock and McCabe (1999). Specifically, half of the surplus generated each month becomes runoff while the remaining half is carried over as surplus to the following month. 

Supdate(month)=Supdate(month-1)+0.5*max(0, 0.95*P - ET-SR)

qupdate = 0.05*P + 0.5*S(month)

All forcing data come from TerraClimate but are run this this modification of the water balance model.