Drainage of Agricultural Lands - Practices of Irrigation and On-farm Water Management - page 372

reclaimed land rationally, and to apply the scarce available financial means to repair
improperly functioning drains.

9.11.2 Evaluation System

Discharge ( q ) and water-head midway between drains ( h ) are two important design
parameters of subsurface drainage. To evaluate the performance of subsurface
drainage, these two parameters, and water-head on top of the drains, are to be
measured and compared with the design (and/or expected) value under the prevail-
ing conditions. Measurement of water table should be carried out every 3-5 days
depending on rainfall amount, ET demand, and hydraulic conductivity. The mea-
surement should be continued for a reasonable length of time (at least one season or
a part), and in several plots.
If the design parameters differed significantly from the designed/expected value,
it indicates that original design criteria are not satisfied. The causes of drain mal-
functioning may include the drains are too widely spaced, drain envelope materials
are not properly designed, changes in hydraulic conductivity of the soil from the
measured/gauged one (may be due to erroneous measurement of K , compaction of
soil due to traffic or natural compaction/settlement). More elaborate measuring set-
up as well as long-term observation may be needed to ascertain the causes of failure
or drain malfunctioning. Under natural rainfall condition, low rainfall can results in
small discharge and low head.
In addition to the piezometer on top of the drain (which measures the water-head
above the drain), another piezometer may be connected to the drain to ascertain
whether the head above the drain is due to entrance resistance or to backpressure in
the drain.
If we consider the drain center as reference level, the total head loss in the system
( h t ) can be expressed as

h t = h m − r 0

where

h m = head in the piezometer midway between drains (or the height of the

groundwater table midway between drains above drain level) (m)

r 0 = outside radius of the drain (m)

The approach flow head loss or the head loss in the vicinity of the drainage sys-
tem (drain pipe and envelope material), h ap , is given by (Rimidis and Dierickx,
2003 ) :

h ap = h v − r 0