فصلنامه حفاظت منابع آب و خاک
سال دوم شماره 4 (پیاپی 8، تابستان 1392)

Quantitative description of soil hydraulic properties is necessary for any scheduling related to soil and water resources conservation. Soil hydraulic properties are important inputs for simulation of water flow and solute transport in soil. Since direct measurement of these properties is time consuming and costly، pedotransfer functions (PTFs) have been widely and successfully used for their prediction. Yet، little efforts have been made for functional evaluation of PTFs predictions for application in simulating transient soil water flow. In this study، soil water retention curve (SWRC) of a clay soil was predicted، using some selected local and global PTFs. SWRC was also measured in the laboratory by direct method. Validity of studied PTFs in terms of prediction of SWRC was examined. By applying both predicted and measured hydraulic parameters to HYDRUS-1D program for simulation of soil water flow، functional behavior of PTFs was quantitatively compared in terms of simulation of water flow in soil. The obtained results indicated that both the selected global PTFs (Rosetta) with root mean square error (RMSE) < 0. 025 cm3cm-3 and some regional PTFs (Ghorbani-1) with RMSE < 0. 014 cm3cm-3 can reasonably well predict SWRC of soil surface. For prediction of SWRC of subsurface soil، the RMSE value of global PTFs was larger than 0. 107 cm3cm-3 and was ranged from 0. 036 to 0. 356 cm3cm-3 for the local PTFs. However، for simulation of soil water flow a different set of PTFs was most efficient. RMSE values of adjusted Ghorbani PTF (S-Gh&H (Adj.)) for simulation of water content of surface and subsurface soils were 0. 025 and 0. 055 cm3cm-3، respectively. RMSE value of Wosten (1997، 1999) PTFs for surface soil was larger than 0. 149 cm3cm-3 and for subsurface soil was less than 0. 058 cm3cm-3. Simulation with the measured SWRC had RMSE values of 0. 013-0. 040 cm3cm-3 for surface and subsurface soils، respectively. It can be concluded that when validating PTFs، one should consider the objective for which the PTFs are assessed. A PTF might be accurate enough for predicting SWRC، but not for particular  (h) range governing the soil water redistribution process. Using such PTFs might lead to large errors in simulating soil water content.

Quantitative perception of soil hydraulic behavior has important impact on optimal soil and water resources conservation. Understanding of this behavior in drip irrigation system is a real task for researchers. The geometry of moisture pattern created by trickles in soil is one of the important criterions in drip irrigation system. Several important parameters such as soil hydraulic functions, drip discharge and irrigation time directly influence the dimensions of created wetting front. The objective of this study was to determine the wetting front dimensions created by point sources. For examining HYDRUS-2D software capability, a T tape drip irrigation method with 2.388 cm/hr discharge and maximum depth of 70 cm was conducted in the Urmia University experimental field. At the end of irrigation experiment, the water distribution condition in soil was determined by gravimetric soil sampling method. When performance of simulation model confirmed to be accurate enough for simulation purposes, six different discharges with trickle function of maximum 30 h, was conducted by the software. Some simple relationships were obtained by using Buckingham π theorem. These equations determinate the depth and maximum diameter of soil by using soil hydraulic conductivity, drip discharge and irrigation time. The obtained correlation coefficients (0.993 and 0.970) and mean absolute error values (1.177 and 1.706) for determining the depth and maximum diameter of soil confirmed the capability of these semi empirical equations for calculating moisture pattern geometry dimensions. Consequently, these derived equations can be used for design and optimal management of drip irrigation system in the studied conditions.

Accurate estimation of evapotranspiration is crucial for optimal conservation of soil and water resources in arid and semi-arid regions. Evapotranspiration is an important component of water balance in arid and semi-arid regions. The objective of this study was to evaluate SEBAL model to estimate ETa in a semi-arid area. The study was carried out in Zanjanrood watershed، Zanjan province، which is dominated by both irrigated and rainfed wheat cultivation. Using MODIS (Terra/Aqua) and TM (Landsat-5) satellite data، the feasibility of using SEBAL model with multi sensors data was investigated from June to July 2007. The estimated daily ETa values which were later obtained by SEBAL model were evaluated based on the two real evaporation pan data. The statistical results indicated that SEBAL model has reasonable potential to estimate ETa of irrigated (RMSE= 0. 49 to 1. 25) and dryland (RMSE= 1. 3 to 2. 48) wheat fields. The SEBAL model evaluates ETa of irrigated wheat field more accurate than dryland wheat field. The results further indicated that the TM images are 2. 5 times more accurate than MODIS images. Furthermore، it can be concluded that the TM sensor located at Landsat satellite can better predict the ETa values than the MODIS sensor located at Terra satellite sensor in the studied area.

Infiltration parameters are the most important indicators of soil quality. Soil water infiltration is one of the key properties for designing irrigation systems، hydrological studies، water resources management، drainage projects and soil conservation practices in watershed scale. The objective of this study was to investigate the effect of land abandonment on infiltration parameters in semi-stepped rangelands located in Karsanak، Chahar Mahal va Bakhtiari province، Iran. For this purpose، five types of land uses including pasture، agriculture، 3-5، 10-15 and 25 years-long abandonment were selected and the infiltrated water was measured in six replicates by tension infiltrometer apparatus. The results indicated that land use changing led to reduce soil organic matters، soil aggregate stability، soil pores connectivity and to disarranging soil natural pores. Consequently، the negative and significant effect of land use changing on parameters of water infiltration was deduced. Average saturated hydraulic conductivity in pastures (7. 4 mm/h) was almost twice of that for agricultural land use (4. 4 mm/h). In addition، the sorptivity experienced 30 percent reductions in agricultural lands compared to pastures. However، because of the land abandonment i. e. restoration of vegetation and macro aggregate formation and increasing aggregates stability، the infiltration process was improved such that saturated hydraulic conductivity of the 25 years-long abandonment improved from 4. 38 to 6. 09 mm/h.

Soil erosion control is very important to natural resources conservationists and managers. For this reason، different methods and practices have been developed and used to control soil erosion. Application of soil amendments is one of the most recent technologies in soil and water conservation. However، different aspects of its application have not comprehensively been considered. Hence، the present research intended to assess the application of polyacrylamide (PAM) as an important soil amendment on soil loss and sediment concentration. The research was conducted under simulated rainfall condition in small plots on the loess soils of Bojnourd City located in Northern Khorasan Province، Iran. Accordingly، the laboratory experiments were conducted in treatments of 0 (control)، 0. 4، 0. 6، 1، 2، 3، 4 and 6 g m-2 of polyacrylamide with simulated rainfall intensity of 72 mm h−1 with 30 minutes duration in three replicates. The collected data on soil loss and sediment concentrations showed that polyacrylamide significantly reduced both soil loss (P=0. 04) and sediment concentration (P= 0. 004). The maximum sum amount of soil loss and sediment concentration were 49. 32 g and 10. 79 g/l، respectively، at control treatment and their minimum were 18. 03 g and 6. 93 g/l respectively، at 6 g/m2 level of PAM application. Also، the results showed that the polyacrylamide had no significant effect on temporal changes of soil loss and sediment concentration during event duration (P>0. 3). The results suggest that PAM، as an available amendment، can be recommended for water and soil conservation purposes.

Erosion Plots are widely used to evaluate the main factors affecting soil erosion. Therefore, understanding the effects of different variables such as spatial scale on their performance is needed. This study was carried out to assess the effect of plot scale on measured runoff and sediment yield at event scale. To represent different spatial scales, plots with seven sizes differing in length (2, 5, 10, 15, 22.1, 25, and 30 m) and width (1, 1, 1.2, 1.6, 1.8, 2, and 2.4 m, respectively) were constructed in Poldasht region, west Azarbayjan province, northwest Iran. For each size of plots, specific runoff and sediment yield were separately measured for 11 runoff producing natural storms during the study period from September, 2010 to September, 2011. The results indicated that the unit area runoff and sediment yield decreases with power form relation as plot area or length increases. Statistical analysis of runoff and sediment yield data showed that, in most cases, there is a significant difference between the results of 10 m or shorter in length, plots and longer plots (p<0.05). However, there was no significant difference between the results of 15 m and longer plots. Based on these results, plots with 10 to 15 m in length were able to produce comparable results with large plots.