فصلنامه حفاظت منابع آب و خاک
پیاپی 3 (بهار 1391)

Drought management is very important for optimal water resources application in arid and semi-arid regions. One strategy to manage drought is to predict drought conditions by probabilistic tools. In this study, total monthly precipitation records related to 33 synoptic stations of Iran during 1976-2005 were used to monitor and predict future drought conditions. Regarding the dry periods greater than six months in the arid regions of the country, the Standardized Precipitation Index (SPI) at 6-month timescale was used for drought monitoring. The first-order Markov chain model was employed to predict drought condition up to 3-step ahead. This model was fitted on the SPI series at all stations of interest, and it was identified that can represent the probabilistic behavior of drought over Iran. The results obtained from drought prediction at 1, 2, and 3-step ahead over Iran showed that the occurrenceof the severe drought (9 percent of stations) or normal conditions (87 percent of stations) is most probable in the future months, regardless of drought condition at current month. Also, drought monitoring based on aerial mean of monthly total precipitation time series over country showed that the trend of drought severity has been increasing in recent years.

One of the most efficient strategies for optimal use of water resources in arid and semi arid regions is the so-called deficit irrigation. Deficit irrigation by reducing number of irrigation or lowering volume of applied water in each irrigation may cause relative yield reduction per unit of farm. This may also cause salt stress in addition to water stress. In arid and semi-arid regions, this may reduce soluble salt leaching from the root zone, especial when growing salt sensitive crops, which causes further yield reduction. It is understood that in these regions leaching salts by providing adequate drainage is the normal practice to cope with water and salinity stresses. In this research using steady state calculation and management salt balance model (SaltMod), the effect of deficit irrigation was studied for four salt tolerant and salt sensitive crops in a long period (20 years) to find out the effect of salt and water stresses on yield of these crops. The salt tolerant crops were wheat and sugar-beet, and salt sensitive crops were corn and sugarcane which are important cultivated crops in Khuzistan province of Iran. Long time changes in soil profile and drainage water was also calculated and simulated. The results of this study showed that in a representative soil of arid-region (central Khuzistan area) with very high salt content upon installing drainage system and required leaching to bring root zone salinity to normal range (ECe < 4 dS/m), there is a good chance to continue wheat and sugar-beet cultivation with normal deficit irrigation (around 20 percent) for a long time. However it was not possible to use normal deficit irrigation for maize and sugarcane for more than a few years without complete crop loss. Increasing salinity of root zone showed the above process very clearly.

 Assessing pollution potential and amount of nitrate contamination in Evan plain as results of husbandry and fertilizer applications is of great importance for this plain. In this study، 16 wells and several surface water sources were sampled. Nitrate concentrations in these samples were analyzed and were compared to drinking water standards proposed by EPA. The results indicated that most amounts of Nitrate can be found at north part of Evan plain that generally are less than 37 mg/lit. To assess the source of groundwater NO3، the NO3/Cl and NO3/K charts were used. High NO3 concentration and low Cl concentration in the samples from north of Evan plain groundwater was related to leaching of chemical fertilizers to groundwater. Spatial distribution of nitrate concentration confirmed the output of both GOD and AVI models where they predicted maximum NO3 concentration at north part of the Evan plain.

Using infiltration models for modeling surface and subsurface flow as well as designing and evaluation of irrigation systems is necessary. Since the parameters used in these equations are highly dependent on the soil types and surface conditions، field tests is a necessary tool for determination of these parameters. Several infiltration models have been proposed for this purpose that are mostly empirical، semi-empirical or theoretical in nature. In order to select an appropriate model for various field conditions، field tests are required. In this study، several empirical infiltration models were compared with one another and with the Philip exact infiltration equation which was obtained by simplifying the numerical solution of general flow equation in porous media. Required data for selecting appropriate model were taken from Oshtorinan plain، Lorestan، Iran to predict cumulative infiltrationvalues. In this area surface soils are dominantly clay. Infiltration tests were conducted in three locations with clay texture، using double rings infiltrometers. Parameters of four infiltration models (Kostiakov،SCS، Philip، Kostiakov-Lowis) were determined. Model evaluation was performed by comparing relative error. The results of this analysis indicated that the Kostiakov model with a mean relative error of 0. 8 was the most appropriate model for estimating cumulative infiltration in the studied area.

Precise prediction and scheduling of plant irrigation water can increase water use efficiency while preventing possible periodical stress. In this study، the performance of BUDGET model in irrigation scheduling was assessed and compared with real data under soybean deficit irrigation water management under two consecutive years 2008 and 2009. The experiment was conducted in the form of randomized complete blocks design in four furrow irrigation treatments each with three replicates. The treatments include full irrigation، (control)، conventional deficit irrigation at 50% and 75% of the soil moisture deficit compensation، and partial root drying treatment at 50% of the soil moisture deficit compensation. The results indicated that the performance modeling indices of RMSE and CRM in 2008 were 3. 91 and -0. 18 and in 2009 were 4. 76 and -0. 11، respectively. The obtained EF index was 0. 69 and 0. 78 in 2008 and 2009، respectively، indicating that the model performed acceptable predictions of crop yield. The overall results revealed that the use of BUDGET model by adjusting crop coefficients in different growth stages، soil and plant characteristics can provide more acceptable outputs. Further، the sensitivity analysis indicated that BUDGET model is less sensitive to irrigation water depth and initial soil water content in the full irrigation treatment.

For conservation of soil and water resources and maintaining sustainability of agricultural productivity، water application under saline conditions should be conducted under precise application of crop leaching fraction. To evaluate leaching requirement and salt tolerance of alfalfa، a lysimeteric study (40 cm diameter and 180 cm height) was conducted through completely randomized design with three levels of salinity irrigation water (3، 7 and 13 dS/m) and three levels of leaching fractions (12، 25 and 50 %) as a factorial experiment with four replicates. After soil packing، the lysimeters were leached out until the salinity of their leaching water reduced to 3 dS/m and the columns were sown with alfalfa seeds (Yazd cultivar). To adjust the irrigation water salinity، natural saline water from Sadouq Salinity Research Station with 14 dS/m salinity was blended with tap water to obtain the required saline water treatments. Crop water requirement including leaching fraction was precisely calculated and supplied to the plants. The amount of collected drainage water was again recorded after 48 hr and its quality indexes were measured. Any deviation from the designed leaching fractions was compensated in the next irrigation event. This way، the precise water and solutes balance was determined. After some irrigation events، depending on salinity and leaching fraction level، drainage water salinity was fitted to the predicted steady state simulation models. The minimum required irrigation cycles to reach steady state conditions was obtained to be 12 cycles which was increased by increasing salinity and decreasing leaching fraction. The overall results showed that implementation of a defined leaching fraction can reduce soil salinity more than what was predicted by WatSuit model. Thus، soil salinity control at a desired level can be obtained by less leaching fraction level which is especially important for optimum water utilization under saline conditions when irrigation is linked to more salt application.

The so-called reduced tillage is one the most important management practices for soil and water conservation. In order to study the effect of reduced tillage on energy productivity and wheat yield in Kalpoush region of Shahrood، a randomized complete blocks design (RCBD) experiment with sunflower-wheat rotation was conducted having five treatments and four replicates. The treatments were consisted of chisel plowing at spring + disking (T1)، moldboard plowing +disking (T2)، reduced tillage with disk harrow (T3)، disk plowing + disking (T4) and chisel plowing at autumn +disking (T5). Sunflower was planted at May 2008 and 2011. After harvesting at October 2008 and 20، all plots were disked. Wheat was planted at November 2008 and 2011 and harvested at July 2009 and 2012. The results showed that there was no significant difference among the treatments (P<5%). In 2009 the maximum (1708 kg/ha) and minimum (1412 kg/ha) crop yield were belonged to T5 and T3 treatments، respectively. In 2011، the maximum (1638 kg/ha) and minimum (1388 kg/ha) yield was obtained from T5 and T2 treatments، respectively. Further data analysis showed that soil water content and wheat yield differences are not significant among the treatments (P<5%). The maximum and minimum soil moisture and wheat yield related to chisel and moldboard ploughs، respectively. Maximum and minimum energy productivity was related to disk harrow and moldboard ploughs، respectively. Chisel plugging increased the wheat yield. Soil inversion index difference was significant among the treatments (P<1%). Maximum and minimum soil inversion index related to moldboard and chiselploughs، respectively. Considering disk harrow advantages and results of this experiment، for wheat production it may be recommended that plowing can be replaced by disk harrow، disk harrowing for seedbed preparation is sufficient، and plowing isn’t required.