RESEARCH ADVANCES IN IRRIGATION METHODS FOR EFFICIENT WATER APPLICATION: SOUTH AFRICA - Management, Performance, and Applications of Micro Irrigation Systems - page 55

lost to nonbenefi cial spray evaporation and wind-drift, in-fi eld conveyance, fi lter and
other minor losses, and the volume of water entering the irrigation system, for a spe-
cifi c period of time. The losses can also be expressed as a depth of water per unit area,
rather than a volume.
Irrigation uniformity is a characteristic of the type of irrigation system used, to-
gether with the standard to which a given system has been designed, is operated and
is maintained. It can also be affected by soil infi ltration characteristics and by land
preparation. The traditional approach to accounting for the distribution uniformity
of the lower quarter (DU lq ) has most likely resulted in the default irrigation effi cien-
cies customarily referred to, for example, that furrow irrigation is assumed to be 65%
effi cient and center-pivot irrigation is assumed to be 85% effi cient. Unfortunately, the
rationale for these assumed effi ciencies, that is, the typical or assumed nonuniformity,
is seldom well considered, and water is often thought to just 'disappear' with the
assumed low effi ciencies. However, once the water-balance approach is applied, it
is realized that the water does not 'disappear' but could contribute to increased deep
percolation, which may eventually appear as return fl ow further along the drain- age
system.
The bottom line is that assuring high irrigation uniformity is of primary im-
portance, and should be the goal of good design and maintenance procedures. It
is very unlikely that low crop yields caused by nonuniform irrigation water ap-
plications will be improved by assuming low irrigation effi ciencies and therefore
increasing the water applications accordingly.
If poor uniformity results in low crop yields, the uniformity needs to be cor-
rected in order to improve system performance. Simply applying more water to
compensate for the part of the fi eld that is being under-irrigated is unlikely to
result in improved crop yields, as large parts of the fi eld will now suffer from
overirrigation, and the risk of long-term problems developing due to a raised water
table will increase. The preferred recommendation in this case would be to deal
specifi cally with the problem of poor uniformity. For planning purposes, the GIR at
the fi eld edge should therefore be calculated as the product of the NIR and system
effi ciency.
4.8 CONCLUSIONSANDRECOMMENDATIONS
Studies and research over 40 years on mainly the engineering aspects of the tech-
niques of flood-, mobile- and micro irrigation contributed to the knowledge base of
applying irrigation methods correctly to improve the efficient application of water. In
particular, the research that was carried out to improve irrigation-water management
from dam-wall release to root-zone application has to a large extent consolidated and
contributed to local knowledge on issues regarding irrigation water-use efficiency.
The resulting approach of 'measure; assess; improve; evaluate,' promotes an investi-
gative approach to improving efficiency, rather than relying merely on water account-
ing.
The main output of this research was the compilation of guidelines for improved
irrigation-water management from dam-wall release to root-zone application. The