Land and Watershed Management - Practices of Irrigation and On-farm Water Management - page 206

The important considerations in the early surveys are as follows:
(a) the topographic features of the land that influence the flow of water by gravity
or the elevation and distance to which water must be pumped
(b) the depths of barriers that can act as obstructions to the constructing of canals,
drains, and other structures or affect grading and land leveling operations
(c) the presence of unstable subsurface materials that may lead to subsidence
problems
(d) the permeabilities of soils on which canals and drains will be constructed and
the associated losses of water for unlined or lined channels
(e) the substratum condition as it affects the installation of permanent structures
such as diversion weirs, storage reservoirs.
(f) soil conditions for installing field and main drainage (i.e., depth to barrier,
nature of barrier, etc.)
(g) the location of dug-wells or tube-wells in respect not only to water but also to
the land that will be irrigated to obtain the best advantages in terms of energy-
saving and topography
(h) the size and shape of potential management units or fields
(i) the positioning of bunds or levees according to topography and changes in soil
texture or other land characteristics, thus improving the efficiency of water use
and productivity
(j) the assessment of basin sizes, furrow lengths, etc. in relation to the earthmoving
costs and the acceptable slopes and microrelief after grading
(k) the matching of water supply and demand and the scheduling of water in terms
of frequency, rate, and duration of application. The design of the canal or pipe
networks to the field and the engineering costs depend on any one or all of these
factors.
In rehabilitation schemes, quite different assessments may be required depending
on whether the scheme is in a rice area, or in an arid, or semiarid area subject to
waterlogging and salinity problems. Other categories also occur in the intermediate
rainfall zones.
In the rice land situation, rehabilitation often involves upgrading the primary,
secondary, and tertiary water supply networks or the installation of improved water
control structures (diversion weirs, measuring devices, storage structures, etc.). The
land evaluator may be called upon to evaluate land suitabilities relating to the
improvement of these engineering works.
In the rehabilitation of saline, sodic, and waterlogged land in arid and semi-
arid areas, surveys are generally required for the engineering works, especially
topographic surveys and groundwater level surveys for the proper location of irriga-
tion and drainage channels. If very high construction costs are implicated, the land
suitability class of the associated land may be downgraded accordingly.
Lot Benching
Lot benching is the grading of lots within a subdivision so that the runoff from each
lot is directed to a stable outlet rather than to an adjacent lot. This practice is most