Figure 6. Spatial distributions of monthly averaged ΔGST for the first year of pumping.
  1. Mechanisms of the temporal nonlinearity in GST variations
The mean ΔGST increase with pumping and gradually reach the dynamic equilibrium after 10 years of pumping for both G1 and G2 (Figures 5a and 5c), showing a nonlinear trend. In G2, the pumping rate is large enough to cause the continuous increase of WTD which becomes lower than the critical depth range after 10 years of pumping (Figure 4b). Such an unsustainable pumping represents the general situation in the global hotspots of GW depletion. For example, WTD in many cones of GW depression in the NCP are of tens or hundreds of meters due to the continuous increase of WTD in the past 50 years (Cao et al., 2013; Fei et al., 2009). The nonlinearity in GST variations for this unsustainable pumping can be explained as follows (e.g., Kollet and Maxwell, 2008). In Figure 7a, soil water is rapidly released from the pores at a small soil water suction while the release becomes difficult when the suction increases. In other words, there is obvious decrease of soil moisture near the land surface when the WTD is small in the beginning of pumping while the variations of soil moisture with WTD are decoupled at a large WTD after a long-term GW pumping. Therefore, variations of GST (or the land surface energy fluxes) are significant at an early stage while weakened with time. Finally, when WTD becomes lower than the critical depth range (1–10 m) with pumping, the variations of GST achieve dynamic equilibrium since the soil moisture and thus the land surface energy flux are not sensitive to variations of WTD anymore.