Drainage of Agricultural Lands - Practices of Irrigation and On-farm Water Management - page 369

seepage from tile drained landscapes. A basic relationship of the model is a water
balance for a vertical soil column of unit surface area, which extends from the
impermeable layer up to the soil surface, and located midway between adjacent
drains.
9.8.2 CSUID Model
The Colorado State University Irrigation and Drainage (CSUID) model is a simula-
tion model that can be used in the design and management of conjunctive irrigation
and drainage systems (Manguerra and Garcia, 1995). The model is based on the
numerical implementation of a quasi three-dimensional finite-difference model,
which solves the Richards equation and the advective-dispersive transport equa-
tion for one-dimensional vertical flow and salt transport in the variably saturated
zone above the water table; and it also solves the depth-averaged Boussinesq equa-
tion and two-dimensional advective-dispersive transport equation for areal flow and
transport in the fully saturated zone below the water table. It is capable of drainage
designing and management.
9.8.3 EnDrain
“EnDrain” is a software tool that calculates drainage discharge, hydraulic head, and
spacing between parallel subsurface drains, pipes, or open ditches; with or without
entrance resistance (Oosterbaan et al., 1996 ) . It shows the curve of water table. The
drain spacing calculations are based on the concept of the energy balance of ground-
water flow. However, the traditional concepts based on the Darcy and water balance
or mass conservation equations are also used. The program allows for the presence
of three different soil layers with different hydraulic conductivity and permeability -
one layer above and two below drain level. “EnDrain” can be used for the reclama-
tion of saline soils. The software is free, and can be downloaded from the site:
http://www.waterlog.info/
9.9 Drainage Discharge Management: Disposal and Treatment
9.9.1 Disposal Options
There are limited number of options available when trying to decide where and how
to dispose of agricultural drainage water into the natural hydrological system.
The common option is to return the water either to natural depressions or lakes or
rivers, or to salt sinks, such as ocean, or to return the water either to the land as part
of the irrigation water supply. The options available to any single project may be
limited because of water quality concerns. Downstream beneficial uses of any sur-
face water body to which drainage water is added must be protected. For example, it