RA 00015.pdf - OAR@ICRISAT

Figure 81.
The BW8 tank partially filled following
intense August storms.
Water-balance Studies
Runoff
In watersheds on deep Vertisols, which were
cropped in the rainy season and cultivated to
graded beds, 7 to 10 percent of the seasonal
rainfall ran off; while under flat-planted conditions
runoff varied from 10 to 17 percent
(Table 65). Runoff from the BW3 B watershed
exceeded that observed from BW1 and BW3 A.
These observations again confirm the hypothesis
that a bed-and-furrow system can be effectively
used to uniformly conserve moisture on the land.
This is also evident from Table 66; in 1973 and
1974, when the BW3 B watershed was cultivated
to a 0.8- and 0.4-percent graded ridge-andfurrow
system, respectively, runoff from this
area was very similar to that on BW1 which has
always been maintained in a 0.6-percent gradedridge
treatment. In 1975 and 1976 when BW3 B
was changed to a 1-percent graded-ridge system
and flat field-bunded conditions, respectively,
runoff was (in relation to BW1) considerably
increased.
Runoff under cropped contour-bunded conditions
(BW6 C/D) was of a magnitude similar to
the bed-and-furrow watersheds; however, in this
situation additional infiltration occurred on only
a small portion of the land and drainage problems
frequently existed due to stagnant water
above the bund. (See Land and Water Management
subprogram.) Runoff on cropped, bedded,
medium deep Vertisols (BW7 B,C,D, and F)
amounted to about 10 percent with somewhat
higher values on steeper (1.0%) slopes.
The largest quantities of runoff were observed
under rainy season fallow, field-bunded conditions;
on BW4 C about one-third of the
seasonal rainfall (and 74% of the 21 Jul and 18,
19 Aug precipitation) ran off. A comparison of
the "fallowed" watersheds BW4 C, BW5 BS and
BW6 B indicates that broad beds in a rainy
season fallow situation affect total runoff to a
similar degree as contour bunds. However, the
disadvantage of inadequate drainage on part of
the land (Fig 82), associated with contour bunds,
does not exist under ridged conditions; planting
can be executed within a few days after rain.
Although this management system is not suitable
for Vertisols at ICR1SAT Center, this finding
may be relevant in those areas where the early
rainy season showers are less dependable and
thus "dry planting" is more risky. Peak runoff
rates observed this year (and also earlier) indicate
that under cropped bedded conditions and under
rainy season fallow, values of 0.05 to 0.1 and 0.1
to 0.15 m 3 /sec per ha, respectively, are obtained.
Except under contour-bunded situations,
runoff on the Alfisols amounted to about 20
percent of the seasonal rainfall (Table 67). On
shallow soils runoff was considerably increased
by beds and furrows (RW1 D); in case runoff is
collected for later re-use, this may be desirable on
light soils as long as erosion is not excessive.
During high-intensity long-duration storms, almost
50 percent of the rainfall was lost under flat
as well as bedded planting. Peak discharge rates
on these soils generally range between 0.15 and
0.20 m 3 /sec per ha.
Erosion
Soil losses measured at watershed outlets are
presented in Table 68. Although the observed
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