The Inter-annual Variability of Controlling Parameter of Catchment Water Balance and Its Semi-empirical Formula Based on the Budyko Hypotheses

Abstract:

The long-term average of the controlling parameter of catchment water balance has been widely reported; however, their inter-annual variability has rarely been quantified. Besides precipitation (P) and potential evaporation(ET₀), the surface condition and seasonality of climate have great impacts on inter-annual variability of catchment water balance, which can be reflected by the parameter w (in terms of Fu’s equation). Two watersheds on the Loess Plateau were thus chosen to quantify their relationships. To diminish the impacts of catchment water storage on water balance, the annual water balance was firstly estimated for each water year from 1981 to 2012. Then, the annual maximum vegetation coverage (M) based on NDVI and the variation coefficient (σ) of daily wetness index (P/ET₀) were used to respectively present the surface condition and the seasonal variations in the coupled water and energy, and further discuss their relationships with w. Results showed that w correlated well with M and σ, then a semi-empirical formula was developed to calculate the key parameter w on annual scale (w=1+5.99×M^1.01×exp (-0.072σ), R²=0.60). The equation was further validated in some other watersheds on the Loess Plateau and proved to be superior in estimating actual evaporation (ET). Finally, the Fu’s equation and the semi-empirical formula for w were combined to quantify the contributions of changes in climate (P, ET₀ and σ) and surface condition (M) to ET variations. Results showed that σ and M accounted for 5.8% and -3.2% of the ET decrease for the period of 1981-1995, respectively; during 1996-2012, the contribution of σ to ET changes decreased while that of M increased by 18.9%, indicating the impacts of surface condition on catchment water balance were strengthened.