How significant is sub‐daily variability of rainfall for hydrological modelling of floods? A satellite based approach to sub‐daily downscaling of gauged rainfall

Abstract

Floods exhibit pronounced sub‐daily variations originating from the rainfall variability during storm events. Accurate flood simulations require sub‐daily gauged rainfall which may not be available in all cases. This study evaluates an approach wherein daily gauged rainfall was downscaled to 3 hr resolution using the Tropical Rainfall Measurement Mission's (TRMM) 3B42 level 3 satellite rainfall product (Rsat). Hydrological performance of the TRMM downscaled rainfall (Rds) was compared with Rsat and daily gauged rainfall downscaled uniformly at 3 hr intervals (Runi) by forcing them individually into a pre‐calibrated distributed hydrological model. Satellite based sub‐daily downscaling of daily gauged rainfall (Rds) was witnessed to be the best alternative for flood analyses in the absence of sub‐daily measurements. Visual and statistical evaluation showed the superiority of Rds in flood hydrograph performance (hourly median values of Nash–Sutcliffe efficiency [NSE] 0.89 and R2 [co‐efficient of determination] 0.96) as well as in full‐length hydrograph performance (hourly overall values of NSE 0.72 and R2 0.69). Between Runi and Rsat, the choice rests on the purpose as the latter was more valuable for flood analysis while Runi was the better choice for achieving water balance and full‐length hydrograph match. The performance of Rds (good), Rsat (moderate) and Runi (poor) shows that a combination of rain gauge data downscaled with sub‐daily information from satellites provides the best option for accurate hydrological modelling of floods. Interestingly, Rds induced an additional lead error of 3.5 hr (median value) in flood peak timing which is recommended as a subject for future research.