o_19f3m1n7k170b14e9u0oqipgu4a.pdf

tration and excess application is required to prevent the
accumulation of salts in the root zone.
Irrigation demand is the amount of water required to meet
the needs of the crop and also overcome system losses.
System losses will consist of percolation, surface water
runoff, and transmission and distribution losses. In addition
to the above losses, the application of water to crops
will include evaporative losses or losses due to wind drift.
These losses may be difficult to quantify individually and
are often estimated as single system efficiency. The actual
efficiency of a given system will be site specific and
vary widely depending on management practices followed.
Irrigation efficiencies typically range from 40 to 98 percent
(Vickers, 2001). A general range of efficiencies by
type of irrigation system is shown in Table 2-6.
Since there are no hard and fast rules for selecting the
most appropriate method for projecting irrigation demands
and establishing parameters for system reliability, it may
be prudent to undertake several of the techniques and to
verify calculated values with available records. In the interest
of developing the most useful models, local irrigation
specialists should be consulted.
2.3.2 Reclaimed Water Quality
The chemical constituents in reclaimed water of concern
for agricultural irrigation are salinity, sodium, trace elements,
excessive chlorine residual, and nutrients. Sensitivity
is generally a function of a given plant’s tolerance to
constituents encountered in the root zone or deposited
on the foliage. Reclaimed water tends to have higher concentrations
of these constituents than the groundwater
or surface water sources from which the water supply is
drawn.
The types and concentrations of constituents in reclaimed
wastewater depend upon the municipal water supply, the
influent waste streams (i.e., domestic and industrial contributions),
amount and composition of infiltration in the
wastewater collection system, the wastewater treatment
processes, and type of storage facilities. Conditions that
can have an adverse impact on reclaimed water quality
may include:
• Elevated TDS levels
• Industrial discharges of potentially toxic compounds
into the municipal sewer system
• Saltwater (chlorides) infiltration into the sewer system
in coastal areas
Table 2-6.
Efficiencies for Different Irrigation Systems
Irrigation System
Potential On-Farm Efficiency 1
(Percent)
Gravity (Surface)
Improved gravity 2 75-85
Furrow 55-70
Flood 40-50
Sprinklers
Low energy precision application (LEPA) 80-90
Center pivot 3 70-85
Sideroll 60-80
Solid set 65-80
Hand-move 60-65
Big gun 60-65
Microirrigation
Drip 80-95
1
Efficiencies shown assume appropriate irrigation system selection, correct irrigation design,
and proper management.
2
Includes tailwater recovery, precision land leveling, and surge flow systems.
3
Includes high- and low-pressure center pivot.
Source: Vickers, 2001.
22