How EFDC calculate Conservation of Mass

I use EFDC+ Test Case “TC-02 Neckar Model” to test the Conservation of Mass. Because it’s a 2-D model and for imcompressible flow, the equation is like the one in Fig.a.I choose time as T = 0.4833337, two cells is L1 = 881 (cell_1) and L2 = 929 (cell_2) (Fig.1). For cell_1, Vx: 0.2695 m/s, Vy: 0.0217 m/s. For cell_2, Vx: 0.2072 m/s, Vy: 0.1030 m/s. So △u = 0.2072 m/s - 0.2695 m/s = -0.0623 m/s; △v = 0.1030 m/s - 0.0217 m/s = 0.0813 m/s.Because the coordinate (which is the center of one cell) for cell_1 is (517826.696 m,5410333.625 m), for cell_2 is (517834.180 m,5410332.500 m), the △x = 517834.180 m - 517826.696 m = 7.484 m, △y = 5410332.500 m - 5410333.625 m = -1.125 m.So I can calculate the partial derivative according to these numbers: (partial u)/(partial x) ≈ (△u)/(△x) = [-0.0623 m/s] / [7.484 m] = -0.008324; (partial v)/(partial y) ≈ (△v)/(△y) = [0.0813 m/s] / [-1.125 m] = -0.0723. These two terms are not summed to 0. Could someone tell me what is wrong?

Could someone teach me how to correctly test the Conservation of Mass in EFDC Explorer? I want to understand it better. Thanks in advance.

I think the best way is to use the Mass Balance Tool in the Model Analysis Tab.

I would like to understand better about how EFDC discretize the Mass Conservation Equation by looking to its simulated results. I tried to verify it. Maybe I should post this question in “EFDC Model” Forum, instead of this part of Forum.