The temporal changes of soil moisture at one subsurface layer where ground water is deep can be controlled by precipitation and plant transpiration. Here I want to quantify the contributions of daily  precipitaiton (P), cumulative daily precipitation (CP), daily transpiration (T) and cumulative daily transpiration (CT) to soil moisture variations at 100 cm (θ). I would like to do this by using multivariate linear regression in SPSS. However, I find that the input variables of CP and CT are highly linear correlated. To my knowledge of statistics, one of them should be removed from inputs. But they should be retained according to physical process of water flux; they represent downward and upward water flux of controling soil water dynamics, respectively. Furthermore, I check that P is not linear correlated with θ (P>0.1). What can I do for these problems?  Thanks a lot for any help in advance.

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