Quantification of the nature, extent, and spatial distribution of salt-affected soils (SAS) for India and the world is essential for planning and implementing reclamation programs in a timely and cost-effective manner for sustained crop production. The national extent of SAS for India over the last four decades was assessed by conventional and remote sensing approaches using diverse methodologies and class definitions and ranged from 6.0 to 26.1 million hectares (Mha) and 1.2 to 10.1 Mha, respectively. In 1966, an area of 6 Mha under SAS was first reported using the former approach. Three national estimates, obtained using remote sensing, were reconciled using a geographic information system, resulting in an acceptable extent of 6.73 Mha. Moderately and severely salt-encrusted lands having large contiguous area have been correctly mapped, but slightly salt-encrusted land having smaller affected areas within croplands has not been accurately mapped. Recent satellite sensors (e.g., Resourcesat-1, Cartosat-2, IKONOS-II, and RISAT-2), along with improved image processing techniques integrated with terrain and other spatial data using a geographic information system, are enabling mapping at large scale. Significant variations in salt encrustation at the surface caused by soil moisture, waterlogging conditions, salt-tolerant crops, and dynamics of subsurface salts present constraints in appraisal, delineation, and mapping efforts. The article provides an overview of development, identification, characterization, and delineation of SAS, past and current national scenarios of SAS using conventional and remote sensing approaches, reconciliation of national estimates, issues of SAS mapping, and future scope.
Abstract. The spatial variability of transport parameters has to be taken into account for a reliable assessment of solute behaviour in natural field soils. Two field sites were studied by collecting 24 and 36 small undisturbed soil columns at an uniform grid of 15 m spacing. Displacement experiments were conducted in these columns with bromide traced water under unsaturated steady state transport conditions. Measured breakthrough curves (BTCs) were evaluated with the simple convective-dispersive equation (CDE). The solute mobility index (MI) calculated as the ratio of measured to fitted pore water velocity and the dispersion coefficient (D) were used to classify bromide breakthrough behaviour. Experimental BTCs were classified into two groups: type I curves expressed classical solute behaviour while type II curves were characterised by the occurrence of a bromide concentration maximum before 0.35 pore volumes of effluent (MI<0.35) resulting from preferential flow conditions. Six columns from site A and 8 from site B were identified as preferential. Frequency distributions of the transport parameters (MI and D) of both sites were either extremely skewed or bimodal. Log-transformation did not lead to a normal distribution in any case. Contour maps of bromide mass flux at certain time steps indicated the clustering of preferential flow regions at both sites. Differences in the extent of preferential flow between sites seemed to be governed by soil structure. Linear cross correlations among transport parameters and independently measured soil properties revealed relations between solute mobility and volumetric soil water content at time of sampling, texture and organic carbon content. The volumetric field soil water content, a simple measure characterising the soil hydraulic behaviour at the sampling location, was found to be a highly sensitive parameter with respect to solute mobility and preferential flow situations. Almost no relation was found between solute transport parameters and independently determined soil properties when non-preferential and preferential samples were considered separately in regression analyses. Future work should concentrate to relate integrated parameters such as the infiltration rate or the soil hydraulic functions to solute mobility under different flow situations.
Ground water table (g.w.t.) levels were measured twice a month for 2 years in 50 observation wells installed inside and outside the two 18-year-old and 350 m apart plantations of Eucalyptus tereticornis (Mysure gum) at DhobBhali research plot located in Rohtak district of Haryana state (north-west India). Throughout the study, the g.w.t. underneath the plantations remained lower than the g.w.t. in the adjacent fields. The average g.w.t. in the plantations was 4.95 m and the average g.w.t. in the control located in adjacent fields was 4.04 m. Interestingly, the spatial extent of lowering of g.w.t. in the adjacent fields was up to a distance of more than 730 m from the edge of a plantation. Drawdown in the g.w.t. developed due to the effect of a plantation was similar to the cone of depression of a pumping well and the drawdown in the g.w.t. developed due to the joint effect of both the plantations was similar to the combined cone of depression of two pumping wells. There was no correlation between soil salinity and the g.w.t. The fluctuations in g.w.t. caused fluctuations in g.w.t. salinity in the plantation as well as in the adjacent fields, but there was no net increase in g.w.t. salinity underneath the plantation. Sinker roots of Eucalyptus tree reached the zone of capillary fringe up to a depth of 4.40 m, indicating that the Eucalyptus trees were absorbing capillary water of the g.w.t. Thus, in shallow g.w.t. areas of semi-arid regions with alluvial sandy loam soils, the plantations of E. tereticornis act as bio-pumps and therefore, we recommend closely spaced parallel strip plantations of this species for the reclamation of waterlogged areas.
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