From a total 3 million ha of Khozestan plain, about 75000 ha are cultivatable and the rest of 2.25 million ha are faced with limitations such as salinification, sodification and water logging. These lands can be cultivated if one spends money for reclamations and also special if management procedures are considered. In saline soils, water absorption by the plant, which is conditioned to osmotic characteristics is scattered due to increase in salinity concentrations and this stops plant growth. In addition, existing ions in the soil such as sodium and chloride creates derangement and toxicity symptoms for plants. Generally, one of the reasons for low yields in irrigated fields in arid and semi-arid areas are due to the tendency of the soil in becoming sodic and saline. So, reclamation of the lands is highly considered in the proposed program. In order to reach a good economical efficiency both in agricultural and horticultural products, these reclamation procedures should be carried out beforehand. Before starting any reclamation program, a key question is put forward: How much water and for how long it is needed to reach the desired amount of salinity in the soil profile?
Determination of the amount of water needed for leaching is based on field tests and the derived curves. But this can be time and cost consuming. So, a better way is to use simulation models. In a proper model, it is possible to consider different managing operations and various situations and compare different scenarios, results and parameters easily and modify or change them where necessary.
Pazira et al. (1998) developed a model to determine
the needed amount of water for leaching the soils in the east-south of Khuzestan.
Soil electrical conductivity before and after leaching, equilibrium electrical
conductivity and soil depth, were used as entering data for this model.
Researchers developed empirical models to determine the needed amount of water for leaching. These models are based on the soil types in the experimental location. Except for the model presented by Reeve, all other models are based on leaching using intermittent ponding method.
In intermittent ponding method, due to the unsaturated situation which is created,
leaching efficiency is higher than continuous ponding method (Cote
et al., 2000). Ben-Gal et al. (2008)
reported that excess in theyield depends on soil salinity when soil irrigated
with saline water. Konukcu et al. (2005) showed
that except for the effects of leaching on plant transpiration rate, salinity
rate in the root zone is very important too.
Shani et al. (2007) developed a mathematical
model to determine water absorption rate of the plants in high soil salinity
rates. Corwin et al. (2007) in an economical
evaluation for leaching methods, compared two kinds of methods and concluded
that sustainable models such as WATSUTT are rather preferable than unsustainable
ones like UNSATCHEM.
Khaksari et al. (2005a) in their study in Chahafzal
area concluded that 80-85% of the salts can be removed from the soil by utilizing
100 cm of water. They also concluded that time intervals leaching periods, play
a major role in leaching process. In another research, Khaksari
et al. (2005b) compared the SWAP model with the LEACHC model. Their
comparisons were based on derived Root Mean Square Error (RMSE), efficiency,
CRM and the coefficient of determination. They showed that LEACHC gives better
results as it uses different interaction transform mechanisms of the soil environments
like absorption, sedimentations and dissolving rate.
Since field experiments are faced with the limitations such as being time and
cost consuming, so, it is substituted by simulating models. These models can
help the researchers in many aspects and gives relatively concise results too
(Droogers et al., 2000).
The main objective of this study was to study the desalination and desodification of Khuzestan plain soils in order to develop a reclamation-rotation program in the sugar cane fields.
MATERIALS AND METHODS
The study area is located in the mid-part of Khuzestan plain and covers an area of about 20167 ha. These lands are restricted by Dez River in the North and from the East; it reaches to Karoon river, while in the south it is restricted to the city of Ahwaz. This research was conducted from 2006 to 2008.
Soil texture is mostly silty clay. The lands of the study area are classified
in S4A4 category by Willcox nemogram (FAO, 1973) while
over 90% of the lands have salinity limitations from large to relatively large
scale. Seven sites were selected by primary investigation of soil salinity maps.
Some physical characteristics of different soil layers before leaching are listed
in Table 1.
Water amount needed for leaching was supplied from Karoon River. Intermittent
ponding method (Loáiciga and Allison, 2007) was
used with six double rings (in a circular area with 10 m in diameter) and total
water depth was 100 cm (4 depths of 25 cm).
||Some physical characteristics of the soil layers before leaching
Soil sampling was picked (NRCS, 1992) with the following
frequencies: 0-25, 25-50, 50-75, 75-100, 100-125 and 125-150 cm. Also, three
replications were conducted in each treatment. Soil samples before and after
leaching were analyzed in the laboratory. Some chemical characteristics of the
different soil layers are listed in Table 2.
Equilibrium salinity/sodicity was measured after infiltration of 100 cm of water from 0-5 cm depth in 3 replications. Using the values in Table 1 and 2, desalinization values were extracted via the following formulas:
and desodification values were extracted via
|ECi and Ecf
||Electrical conductivity before and after leaching operation
(in this study EC used to describe of the salinity)
|ESPi and ESPf
||Exchangeable sodium before and after leaching operation
||Equilibrium electrical conductivity
||Equilibrium exchangeable sodium
Four models (exponential, power, reverse and logarithmic) were fitted on all computed X and Y. Then efficiency coefficient, standard error at the significance level of 1% for each model was compared. Among them, the best fitted model with the highest efficiency were selected and presented to be implemented.
Needed water for leaching to decrease soil salinity was determined using the
fitted model and both scenarios were considered.
||Some chemical characteristics of soil layers before leaching
for the soils in the permanent sugarcane field of the study area was presented
while in the scenario I, regarding initial salinity of the layers of 0-25, 25-50,
50-75, 75-100, 100-125 and 125-150 cm which are 104.7, 40.9, 34.4, 32.6, 30.7
and 29.6 dS m-1.
Considering the noted status, a simulated model called leaching process was developed and both scenarios were considered. Needed water for initial leaching necessary to decrease soil salinity down to the suitable threshold for barley and sugarcane cultivation was determined using the simulated model. The following data were used in the model as input:
||Initial salinity of different soil layers
||Leaching efficiency coefficient (0.27)
||Equilibrium soil electrical conductivity (ECeq = 2.2 dS m-1)
||Soil moisture content (0.31) as soil moisture at this level
is a little more than the field capacity, desalination would be carried
||Final electrical conductivity (ECf)
RESULTS AND DISCUSSION
Desalination and desodification of the soils: In order to implement desalination and desodification of the soils, empirical models were considered to develop a new model for the study area. An exponential model with the efficiency coefficient (R2) equal to 0.735 and Standard Error (SE) of 0.68 significance level at 1% was chosen as the best fitted model as relation (4).
where, X and Y were explained as relations 1 and 2.
Leaching efficiency of the soil profile (f) and soil moisture content (θv) were derived from the Eq. 5:
||Soil desalinization curve in the study area
As soil texture of the study area is heavy, (f) would be equal to 0.27 and the soil moisture content was estimated to be equal to 0.31. When the desalinization process was considered according to the field experiments, the Eq. 6 was developed with R2 = 0.758.
where, X and Y were explained as relations 3 and 4.
Final equation was conducted as relation 7.
Desalinization and desodification curves were presented in Fig. 1 and 2. Using these results, net water depth (Dlw) needed for decreasing soil salinity and exchangeable sodium can be determined.
The results of comparison between existing empirical models and the proposed
model (Eq. 4) were listed in Table 3. Initial
salinity in the depth of 150 cm is equal to 45 dS m-1 and final salinity
would be around 8 dS m-1. It is possible to conclude that models
of Pazira et al. (1998), after the proposed model
of this research consume less water in the leaching process, respectively.
||Results after application different empirical models
|*Based on amount of needed water
||Soil desodification curve in the study area
Also other models such as Verma and Gupta, Reeve and Dieleman can not be fitted
at all regarding to the soil and water characteristics of the study area. Table
3 shows the results after the application of the proposed model along with
its order in the fitting status in the study area.
Proposed reclamatin-rotation program
Scenario I: Based on the proposed model, the amount of water needed to leach for removing the salinity out of the 25 cm layer and reaching to final electrical conductivity equal to 8 dS m-1 (where the yield efficiency for barley is 100%), was estimated to be around 37.5 cm. As deep percolation in the field ponding was estimated about 1.3 cm day-1, maximum needed time for percolation of 37.5 cm water would be 29 days.
Evaporation during summer is very high. For example, data derived from class
A evaporation pan shows values of 521.7 and 410.7 mm in August and September,
respectively, while during autumn season, they reach to 283.3 and 167.8 mm in
October and November, respectively. So it is better to implement leaching operation
in the months with lower evaporation rate. Calculations shows that during percolation
time, average 107.2 mm evaporation occurs, so total gross needed water for leaching
would be 51.7 cm.
||Salinity changes in the soil profile after initial leaching
It is recommended to use 4 alterations (13 cm depth water
for each). Three days are considered for water percolation between the time intervals.
Therefore, it is proposed to start leaching procedure on October 1 and end it
on November 22. Figure 3 shows salinity changes of the soil
layers after initial leaching.
Figure 3 shows after leaching operations, final salinity
in the lower layers of the soil profile has been increased. This is a cause
of leaching procedures of the higher layers and salt transformation to the lower
layers. So, after leaching operation, when soil moisture reaches to the field
capacity, barely should be cultivated in late November. At this time, Barley
seeds can use normal precipitation effectively. Water requirement for barely
in the study area was estimated about 3377 m3 ha-1 (with
the efficiency of 53%). Out of this amount, a total of 12.84 cm is removed from
the root zone as deep percolation occurs and so it causes more decrease in salinity
of the soil layers too. Figure 4 shows salinity changes of
the soil layers after a percolation of some 12.84 cm over 160 days since barely
cultivation. During growing seasons, salinity in the soil profile may be increased
up to 16 dS m-1. Considering the salinity of the applied water (1.37
dS m-1), leaching requirement (LR), was determined (0.035), so that,
the soil salinity during cultivation in the first soil layer, can be controlled
at 8 dS m-1.
||Salinity changes in the soil profile after barely cultivation
||Salinity changes in the soil profile after initial leaching
Regarding to deep percolation rate causing by barely cultivation, there is
no need to consider leaching requirement in the irrigation process.
In the sodification process, all procedures are similar to the desalinization ones. For instance, exchangeable sodium the layer of 25 cm of the soil profile is about 52.5% and equilibrium sodification of the area is 4.2%. While proceed the scenario I, it would be decreased down to 17.15. During the soil and land reclamation, it is recommended to cover the fields with plant residuals especially during the first year of the program. As plant residual helps preventing salinity to return to the soil surface.
Considering the above noted statements, reclamation program is including initial leaching, barley cultivation and using plant residuals and finally an irrigation process before the bed preparedness for sugarcane cuttages.
Scenario II: Soil reclamation procedures should be ended before planting sugarcane cuttages take place. Using the proposed model, needed fresh water depth for a decrease in dissolved salts from 104.7 dS m-1 down to 3.4 dS m-1 (at this salinity rate, the yield efficiency would be 90%) in depth 25 cm was determined. It was estimated to be around 81 cm. As deep percolation is about 1.3 cm day-1, the needed time for the percolation of the mentioned amount would be maximum 63 days. Due to high evaporation during April to late January in the region, it is recommended to implement initial leaching from February to mid-May. During the percolation period (63 days), average evaporation would be about 273.7 mm. By adding evaporation rate with 5.74 cm water depth needed to compensate soil moisture deficit and subtracting 64 mm rainfall from net water depth, total applied water depth for leaching layer of 25 cm would be 108 cm. It is proposed to use this amount in 4 alterations (27 cm in each). So, starting point of leaching operation would be on January 20 and it would be ended on April 23. Figure 5 shows final salinity in the soil layers profile after initial leaching.
After initial leaching, it is recommended to use a heavy disk to break the probable existing crusts. This would minimize capillary current and then evaporation from the soil surface would be decreased. If during the fallow period (even after heavy disk), still surface evaporation happened, salinity of the layer of 25 cm has been increased up to 3.68 dS m-1. As during soil treatment for planting sugarcane cuttages, irrigation is implemented, the salinity of soil surface layer is estimated to be around 3.4 dS m-1. Besides, continuous leaching via deep percolation is still in progress and it would lower it in the soil profile. Considering the above noted, we can conclude that soil reclamation program using scenario II is including: initial leaching, heavy disk and then an irrigation before preparing the bed for planting sugarcane cuttages.
It is quite clear that scenario (I) is a better choice because barley cultivation would remove the remained salts and then soil profile would reach a relevant and bearable amount of salinity so that sugarcane can grow easily. Also consumed water during reclamation procedures in this scenario is much lower compared to the other scenario while the yield is higher as well. It must be noted that the continuation of the soil productivity and prevention from a recurrence of salinity concentration is conditioned by the implantation of proper management and depends on relevant utilization of the soil and water sources.