Time scale and influencing factor variations of water and heat fluxes in sandy land

Abstract

Abstract The transfer process and variation characteristics of water and heat fluxes play an important role in both the ecosystem function and the climatic patterns of arid and semi-arid areas. However, both water (latent heat flux, Fe) and heat (sensible heat flux, Fh) fluxes, and the factors impacting each, operate on different time scales. Therefore, the wavelet transformation was used to reconstruct water and heat flux data measured by an eddy covariance system installed in Horqin sandy land, China. The distribution and response regularity of water and heat fluxes in response to control factors were then analysed across different time scales. The results indicated that the responses of Fe and Fh to controlling factors were similar on 30 min and daily scales, to which the energy terms of net radiation (Fn) and available energy (Fa) showed an extremely significant positive contribution. Removing the interference of the energy terms, Fe and Fh are strongly correlated with the vapour pressure deficit (VPD) and air temperature (Ta) at the daily scale. The water conditions of VPD and relative humidity (RH) were the main influencing factors at the monthly scale. At the seasonal scale, Fe was positively correlated with Fa and Fn but negatively correlated with soil heat flux (Fg), RH and wind speed (WS). On the contrary, Fh was positively correlated with RH but negatively correlated with Fn. At the annual scale, Fe and Fh showed a significant positive correlation with all factors. By the increasing time scale, the cross-correlations between various hydroclimatic processes become better. Generally, at the daily and annual scales, there is a high correlation between Fe, Fh and the control factors, and a pronounced periodicity is exhibited. Another factor that is known to cause such effects is the so-called long-term persistence (or else long-range dependence or the Hurst phenomenon). These results suggest that the time scales of flux and impacting factors should be considered when undertaking water and heat flux analyses of sandy land and other ecosystems.