Ditch system

A mass balance of ditch system is used to represent the human dominating process. The mass balance equation can be described as:

D_o = D_i - D_e - D_g (3)

D_oditch outflow to irrigation area D_itotal ditch inflow D_eevaporation from ditch D_gditch penetration to groundwater where, D_o is ditch outflow to irrigation area, D_i is total ditch inflow, D_e is evaporation from open ditch water surface and surrounding saturated zone, and D_g is ditch penetration to groundwater. Total ditch inflow (D_i) consists of net water diversion from the river main stem and artesian that was used for irrigation. The water diversion from the river main stem was obtained from ARAA. The spring data from the investigation in 1999 was used for the whole study period. The ditch loss was assumed to be in direct proportion to the ditch inflow, and the proportionality constant was identified according to the actual situation of the ditch system. One part of ditch loss was arbitrarily considered to recharge groundwater (i.e. D_g) with a ditch penetration coefficient fluctuating at 0.6-0.8 in this study because the water table was very shallow, and the remaining part (i.e. D_e) was considered to evaporate from the open ditch water surface and the surrounding saturated area. The ditch penetration coefficient highly depends on underlying soil properties, water table depth, and vegetation nearby the ditch. Further research is needed to better understand the ditch loss.