**5. Conclusions**

**0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8**

116 Agroecology

**hs (m)**

0.0

0.2

0.4

0.6

**hs (m)**

0.8

1.0

1.2

**0 4 8 12 16 20 Time (d)**

**Figure 10.** Decrease of the midpoint water table at different separations between drains under a drain depth of 2.00 m.

**Figure 11.** Decrease of the midpoint water table at different separations between drains under a drain depth of 1.50 m.

0 4 8 12 16 20 **Time (d)**

L = 25 m

L = 20 m

L = 15 m

L = 10 m

L = 5 m

L = 25 m L = 20 m

L = 15 m

L = 10 m

L = 5 m

The irrigation in the arid and semi-arid regions to sustain agricultural production against the unpredictable of the rainfall have resulted in the double problem of salinity in many hectares of good agricultural land. Subsurface drainage systems are used to control the depth of the water table and to reduce or prevent soil salinity.

The advection-dispersion equation was solved in order to model the temporal evolution of the concentration of salts removed through an agricultural drainage system with the method of finite differences. The solution requires the values of the flow of water previously obtained from the solution of the Boussinesq equation. The hydrodynamic characteristic were obtained by the inverse problem from the depth drained.

The optimization of the accumulated mass which gave better results in terms of mean square error criterion between the theoretical and experimental values, since it is a property integrated in the time and concentration observed at specific levels. The solution presented coupled to the Boussinesq equation, satisfactorily reproduced the measured data, both in the short time where the change in concentration was high, as in long times where the concentration values tended to an asymptote. This asymptotic value of the concentration depended on the distance between drains of the drainage system.

Finally, the solution of differential equations of transfer processes of water and solute trans‐ port, and hydrodynamic characterization of the soil in an agricultural drainage system, will be a useful tool for designing new systems for the optimal development of crops according to their water needs and the degree of tolerance to salinity. In addition, this study can be help us for quantify crop yield reductions due to salinity on irrigation areas, in order to prevent future problems such as food shortages.
