Water Balance and Salt Balance

There are four major irrigation blocks and some minor blocks in the farm, and the irrigation season starts at the end of April and ends at the end of August. One irrigation block was selected for the study on water and salt balance. In this block, an area of 350 ha was under cultivation for rice, whereas an area of 330 ha was under cultivation for alfalfa. Since this block was recently developed, the area of abandoned fields was only 10 ha.

Water and salt balance was analyzed to clarify the influence of water management on salt accumulation, for developing a proper water management technology. Water balance in the block was analyzed by measuring flow discharge both at the inlet of the irrigation canal supplying water to the block and at the outlet of the main drain from which drainage water from the farmlands is collected.

The water balance situation in the block for 89 days during May 19 to August 16, 1997 shows that, during this period, 18 million m3 of water were introduced to the block. Of this amount, a little less than 9 million m3 of water (a little less than 50%) was consumed and the rest, a little over 9 million m3 of water (a little over 50%), is drained off to the river. When the water volume is converted to water depth by dividing by 680 ha of the total area of the irrigation block, the amount of water intake to the block is equialent to 30 mm/d, water consumed in the whole block is approximately 15 mm/d, and the balance of 15 mm/d of water is drained out. Thus, a large quantity of water is introduced, consumed and drained in the block, notwithstanding the water-scarce arid land. The water consumption in the block consists of crop consumption for rice and alfalfa,

Table 9.1. Some physical and chemical properties of the two soils

Deserted field

Table 9.1. Some physical and chemical properties of the two soils

Deserted field

Depth

Clay fraction

PH

ECe

Major ion content (cmo

l/g

SAR

(cm)

(%)

(dS/m)

Ca2+

Mg

Na+

Cl"

SO42"

G-5

13.5

7.1

99.3

G.3

22.7

97.8

1G3.G 18.1

2GG.6

2G-4G

17.3

7.1

1GG.G

G.9

6.3

17.6

18.1

6.6

63.2

6G-8G

37.2

7.6

91.2

1.5

12.6

31.2

3G.5

15.5

57.6

Alfalfa field

Depth

Clay fraction

PH

ECe

Major ion content (cmo

ljkg)

SAR

(cm)

(%)

(dS/m)

Ca2'

H Mg

Na+

Cl"

SO42"

G-5

41.8

7.6

24.2

1.1

4.6

11.5

6.7

1G.6

4G.3

2G-4G

33.2

7.7

16.5

1.1

2.9

8.1

7.1

7.6

35.2

6G-8G

27.2

6.9

16.1

1.2

3.G

7.6

5.6

7.2

39.4

seepage and percolation in farmlands, ground water outflow to the adjacent areas and canal seepage. Since evapotranspiration (ET) from paddy rice is 8-10 mm/d at maximum, and ET from alfalfa is less than that from rice, excessive water results in ground water rise, due to inadequate functioning of the existing drainage systems.

Water quality analysis was carried out at the farm, and its spatial as well as seasonal changes were identified. The EC shows higher values on the downstream side as compared with that of the upstream side in the irrigation and drainage system. For instance, at the beginning of May, EC was 1.3 dS/m in the Syr Darya River, 1.4 dS/m at the head of the main canal, 1.5 dS/m in the rice field of the block, and 2.6 dS/m at the end of the drain, respectively. Since water quality becomes worse in the Syr Darya River in the latter part of irrigation season, the values of the EC increase throughout the water system in the latter stages of crop growing.

The amounts of salt inflow and outflow were calculated by multiplying the total dissolved solids (TDS) by the observed flow discharge. TDS was estimated using the linear relationship between EC and TDS. Salt balance in total volume for 89 days from May 19 to

August 16, 1997 showed that total salt inflow to the block amounted to 26,000 tons (38 ton/ ha or 427 kg/ha/d), whereas total salt outflow was 22,000 tons (32.6 ton/ha or 366 kg/ha/d). Thus, the balance amounting to 3,700 tons (5.4 ton/ha or 61 kg/ha/d) remained in the block. This can be regarded as the amount of the annual accumulation of salt. The places where salt accumulation occurs are not rice fields, but adjacent areas outside the block and alfalfa fields where percolated water is supplied from rice fields. Especially, salt accumulates in soil layers where water is supplied by capillary rise from the ground water. This tendency could be explained by variations in ground water level and EC value of the alfalfa field located at 300 m from the edge of the paddy fields. Ground water rose from the initial level of about 2 m depth below the surface before the irrigation season to a maximum level of 1-1.5 m. The initial EC values of 6-7 dS/m before irrigation decreased quickly to a value of 1.5 dS/m, equal to the EC of irrigation water, and increased gradually up to 6.5 dS/m at the end of irrigation season. Ground water levels as shallow as 1-1.5 m promote evaporation due to capillary rise and become a continual source of salts to the crop root zone. Although a high ground water level has the advantage of providing moisture supply for those upland crops without any surface irrigation facility, the salinity problem cannot be controlled without lowering the ground water table to the permissible depth of at least 2 m in most soils.

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