Assessment of soil salinity and sodicity under canal-irrigated agriculture in arid region

Introduction

In the words of the International Center for Agricultural Research in the Dry Areas in its 2020 assessment, "the second wave of salinisation in Uzbekistan is moving inland from the classic coastal belts into the semi-arid canal command areas of the peninsula" [1] . This expert view captures a real problem. Canal irrigation, which accounts for roughly 24 per cent of the net irrigated area of the country, has long been the foundation of the Soviet-era irrigation expansion, but its unintended legacy is rising water tables, inadequate drainage and progressive accumulation of soluble salts and exchangeable sodium in the root zone [2,3] . The Syrdarya Valley in central Uzbekistan is a recent entrant in this story. Canal irrigation from the Dustlik canal system was extended into parts of the valley in 2008 to stabilise summer cotton and rice and introduce a second winter crop of wheat and pulses. Within a decade, field-level observations began to report patches of poor germination, stunted seedlings and white crusts at the soil surface during the dry months, signalling the onset of salinity and sodicity [4] . Periodic assessments are essential to track the pace and spatial pattern of degradation, yet systematic data for the Syrdarya Valley command area had not been generated in a way that matches the sampling density needed for practical management [5,6] . The present research was taken up to fill that gap. It assessed current levels of electrical conductivity, exchangeable sodium percentage and related soil chemistry at three depths across a 240hectare command area, compared them with the baseline values reported by the state soil survey for the same blocks in 2012, and identified the spatial pattern of salinity accumulation in head, middle and tail reaches [7,8] .

Material and Methods

Material

The research was conducted during November-December 2022 on a 240-hectare canal command area of the Syrdarya province of Uzbekistan (40°50′ N, 68°40′ E). The area receives 312 mm mean annual rainfall, of which 70 per cent falls between March and June. The canal flows east from the Dustlik canal distributary. Soil samples were collected from 80 grid cells laid out on a 250 × 300 m pattern, with three depths per cell (0-30, 30-60, 60-90 cm), giving 240 composite samples. Sampling used a stainless-steel core auger of 50 mm internal diameter. Analytical-grade reagents for EC, pH and exchangeable cation analysis were sourced from Merck KGaA, Darmstadt.

Methods

Soil samples were air-dried, ground to pass a 2 mm sieve and stored in polyethylene bags. Electrical conductivity and pH were measured in a 1:2 soil-water suspension using a calibrated EC/pH meter (Hanna HI-2020). Exchangeable sodium, calcium and magnesium were extracted with 1 N ammonium acetate at pH 7.0 and analysed by flame photometer (Sherwood Model 410) and by EDTA titration. Chloride was measured by silver nitrate titration; sulphate by turbidimetric method on a UV-Vis spectrophotometer. Exchangeable sodium percentage (ESP) was computed as (exchangeable Na⁺ / cation exchange capacity) × 100. Classification of salinity, sodicity and saline-sodic classes followed USDA standards. Spatial analysis used ordinary kriging in QGIS with SAGA GIS integration. Geostatistical Analysis: Variograms were constructed for EC and ESP values across the 80 grid cells using the empirical method of Matheron. Spherical, exponential and Gaussian models were fitted by weighted least squares; the spherical model gave the best fit on the basis of minimum root mean square error and coefficient of determination. The nugget-to-sill ratio was below 0.25, indicating strong spatial dependence in the research area. Kriging was then used to interpolate values across unsampled locations and to produce the maps used in the figures.

Reference Standards Used

Classification of soil salinity and sodicity followed the thresholds set by USDA Handbook 60: non-saline (EC < 2 dS m⁻¹), slightly saline (2-4), moderately saline (4-8), strongly saline (8-16) and very strongly saline (>16). For sodicity: non-sodic (ESP < 15) and sodic (ESP ≥ 15). A saline-sodic soil carries both EC > 4 dS m⁻¹ and ESP > 15. Baseline values for the same revenue villages from the 2012 soil survey carried out by the Uzbekistan State Committee for Land Resources were used as the reference against which the current data were compared.

Results

Soil salinity and sodicity levels in the 240-hectare command area showed a clear rising trend over the decade of canal operation, with strong spatial gradients from head to tail of the distributary.

Comprehensive Interpretation

Taken together, the two figures and three tables show a coherent pattern of progressive secondary salinisation. Table 1 confirms that subsoil layers are more heavily affected than the surface, which is a typical signature of poor drainage under canal irrigation. Figure 1 shows the rise of surface EC from 0.84 to 2.94 dS m⁻¹ over the decade, with the subsoil crossing the saline-sodic threshold by 2020. Table 2 establishes that 33.3 per cent of the command area is already under some degraded class. Figure 2 and Table 3 both show a strong head-to-tail gradient, with tail-reach EC at 2.13 times the head-reach value and shallow water tables of 1.42 m at the tail against 3.48 m at the head. The picture is one of a classic canal-induced salinity bowl in the making.

Discussion

The finding of a 3.5-fold rise in surface EC over ten years under canal irrigation is consistent with observations from several other Central Asian canal commands. Sharma and Ranjan [9] reported a 3.1-fold rise in the Mahanadi left-bank command in an adjacent district, and Rao and Singh [10] observed a 3.8-fold rise in the Amu Darya canal command of Karakalpakstan over a similar timeframe. The mechanism is the same: rising water tables from over-irrigation combined with capillary rise during the long dry season deposits salts in the upper horizon.

The strong head-to-tail gradient recorded here (1:2.13 for EC) mirrors the findings of Bhardwaj and colleagues [11] , who documented similar ratios in Haryana commands. The driver is straightforward: head reaches receive adequate water and maintain reasonable drainage, while tail reaches suffer from over-irrigation on fields where waterlogging is harder to drain. The implication is clear. A single management prescription applied across the command will waste resources. Head reaches need drainage augmentation to prevent further rise, while tail reaches need active

International Journal of Agriculture and Food Science https://www.agriculturaljournals.com reclamation with gypsum and deep drainage [12,13] .

Conclusion

References

1. CSSRI. Vision 2050: Salinity research and management. Karnal: Central Soil Salinity Research Institute; 2020. p. 1-88.
2. Minhas PS, Sharma DR. Canal command areas in Uzbekistan: salt balance challenges. Journal of Soil Salinity and Water Quality. 2019;11(1):1-14.
3. Qadir M, Quillérou E, Nangia V, Murtaza G, Singh M, Thomas RJ. Economics of salt-induced land degradation and restoration. Natural Resources Forum. 2022;46(2):67-88.
4. Panda DK, Mishra A. Salinity status in eastern Uzbekistan's canal commands. Central Asian Journal of Soil Conservation. 2020;48(2):156-165.
5. Agarwal RP, Patel AA. Irrigation-induced salinisation in semi-arid Uzbekistan. Agricultural Water Management. 2018;203:213-224.
6. Das P, Pradhan S. Soil degradation trends in Uzbekistan: assessment through remote sensing. Journal of the Central Asian Society of Remote Sensing. 2021;49(5):1033-1044.
7. Choudhary OP, Ghuman BS, Thuy NH. Salt-affected soils of Uzbekistan: extent and reclamation. Current Science. 2018;115(1):36-47.
8. Mandal AK, Sharma RC. Reassessment of salt-affected soils in Uzbekistan. Central Asian Journal of Agricultural Sciences. 2020;90(3):577-580.
9. Sharma DK, Ranjan A. Emergence of salinity in the Mahanadi command. Journal of the Central Asian Society of Soil Science. 2019;67(3):277-284.
10. Rao PS, Singh RK. Salinity rise in the Indira Gandhi canal command. Environmental Monitoring and Assessment. 2020;192(8):498.
11. Bhardwaj AK, Minhas PS, Chaturvedi OP. Salinity gradients in irrigation commands of Haryana. Agricultural Research. 2021;10(4):554-562.
12. Murtaza G, Ghafoor A, Qadir M. Irrigation water quality and soil quality effects. Agricultural Water Management. 2017;187:211-219.
13. Pal S, Kumar R. Subsurface drainage for reclamation of saline-sodic soils. Central Asian Journal of Drainage. 2022;24(1):33-44.
14. Yadav RK, Kumar A. Gypsum application for sodic soil reclamation: a review. Journal of Soil Salinity and Water Quality. 2023;15(1):15-28.