An object C is constituted of A and B. Their effects on a variable X (e.g., soil moisture) are denoted as XC, XA, and XB, respectively. Furthermore, there is synergy effect on the variable X between the two parts of A and B, i.e., XS. Then we have XC = XA + XB + XS. Moreover, each of them are functions of time t. My question is how to create a model to describe the synergy effect XS. Thank you in advance.