Water Resources Management - Practices of Irrigation and On-farm Water Management - page 155

Evaporation from open water surfaces can also be reduced by reducing the area
of the water surface. For small surface storage facilities, this can be achieved by
storing the water in deep, small reservoirs instead of in shallow, large reservoirs.
For larger facilities, several ponds or compartmentalized ponds have to be available.
When water levels in the ponds begin to drop, water is then transferred between
ponds or between compartments so that only one or a few deep ponds are kept full
while the others are dry, thus minimizing the water surface area per unit volume of
water stored.
If the ponds are unlined, the effect of water depth on seepage loss from the pond
must be taken into account. From a hydraulic standpoint, increasing the water depth
would increase seepage the most.
From Crop Field Evaporation from soil is reduced by dryland farming techniques
that are aimed at conserving water in the root zone during the fallow season for use
by the crop in the next growing season. The main strategies are weed control, tillage,
and leaving the stubble or other crop residue in the field during the dry or fallow
season. Weed control prevents transpiration losses. Tillage is primarily needed on
heavy soils that may crack during fallow and lose water by evaporation through the
cracks. The purpose of the tillage, then is to close the cracks. Sands and other light-
textured soils that do not crack are “self-mulching” and do not need tillage. Leaving
the stubble or crop residue on the field during fallow periods reduces evaporation
losses from the soil by lowering soil temperature and reducing wind velocities close
to the soil surface.
In the Northern Great Plains of the United States, these dryland farming tech-
niques reduce evaporation losses by about half the annual precipitation. Thus, if
the precipitation is 38 cm/yr as in eastern Colorado, dryland farming techniques
conserve about 19 cm of water per year.
Finally, evaporation of water from soil surfaces can be reduced by reducing the
extent of wet areas from which water evaporates. In irrigated fields with incomplete
crop covers (row crops in the beginning of the growing season, vineyards, orchards),
evaporation from soil can be reduced by irrigating only the areas near the plants and
leaving the rest of the soil (surface or subsurface). This can be accomplished, for
example, with drip irrigation systems (surface or subsurface).
Mulching Mulching with crop residues during the summer fallow can increase soil
water retention. Sauer et al. ( 1996 ) found that the presence of crop residue on the
surface reduced soil water evaporation by 34-50%. Straw mulching can be easily
implemented by local farmers and can be extended in the regional scale because
material is most easily accessible, is available at low cost, and does not contaminate
the soil.
Water Loss Minimization
Water loss in the conveyance system can be reduced through canal lining. A range
of materials are available for that purpose. On-farm water loss (seepage through
the borders of the plot) can be reduced through minimizing holes and proper