Modeling of Soil Salinity variation in predicting Land Suitability in some of wheat and barley farmlands in Neyriz region, Fars Province

Introduction One of the objectives of land evaluation method is determining the land suitability degree and class in case of making any changes, including causing elimination or limitation. Thus, as an example, if it could be possible to predict changes in soil salinity for the future, any changes in land suitability class can be investigated based on the predicted variations over time.The most important crops in Neyriz area are wheat and barley. Unfortunately, over the past two decades, improper agricultural management caused reduction and salinization of irrigation water in this region. To this end, the present study was performed to investigate the possibility of changes in the class or degree of land suitability in case of variations in soil electrical conductivity due to irrigation with saline water in Neyriz, for the next 10 years.
Materials and Methods In three soil map units in three regions consisting of Deh-Fazel, Tal-Mahtabi and Nasir Abad, wheat and barley fields were selected and representative pedons were excavated, described and classified. Soil and water samples were obtained and necessary analyses and soil humidity and salinity, hydraulic conductivity and bulk density and water electrical conductivity were determined. Crop yields were evaluated by 1×1 quadrate, soil surface layer hydraulic conductivity was carried out by guelph permeameter and the volume of irrigation water was measured according to pipe discharge in each farm. Soil retention curve was calculated for all soil layers using sand box and pressure plate. van Genuchten equation parameters were gained using RETC software. Afterward, solute transport modeling was done using the software Hydrus and its results were validated using four statistical parameters including Coefficient of determination (R2), Root Mean Square Error (RMSE), Model efficiency (EF) and Coefficient of Residual Mass (CRM) to investigate the possible variation in soil salinity during the next 10 years, the data of the studied period of the crop year between 1392 and 1393 was repeated for 10 years. Qualitative and quantitative land evaluation was performed by standards methods. Finally, the Hydrus results were compared with salinity maps of Neyriz area which were calculated and obtained in the previous research from Landsat images bands for the past 20 years.
Results and Discussion Based on the results, climate suitability class in Neyriz area was suitable (S1) for wheat and relatively suitable (S2) for barley. The limiting factor for barley was the average of maximum temperature in the coldest month for barley. The soil suitability class was suitable (S1) for both crops (wheat and barley) in all farms. Therefore, the land suitability in the studied farmlands was S1 for the wheat and S2 for the barley. Results also revealed that the values for potential production were 10723 and 8677.5 Kg(grain)ha-1 for wheat and barley and for critical production were 1167 and 1297.6 Kg(grain) ha-1 for wheat and barley, respectively in the farms. Amongst the farmlands, only a barley farm which was located in Tal-Mahtabi had the S1 quantitative suitability class and others had S2. The results also showed that if all other conditions like volume and the quality of the irrigation water, precipitation, temperature and evaporation remain constant over the next 10 years, land suitability class will not change but land suitability degree will decrease gradually over time. The validation of the Hydrus model, based on the RMSE values, revealed that the predicted soil salinity and the observed value were very similar and the model had good ability in estimating and modeling soil salinity in the studied area. Comparing the results of modeling and soil salinity maps over the last 20 years have confirmed this trend. Based on the satellite salinity maps, the soil salinity of the studied fields has increased slightly from 2 to 4 dSm-1 between the years 1374 and 1393. Hence it can be concluded that the prediction of Hydrus model about gradual rise in predicted soil salinity and land suitability degree during the next 10 years is acceptable.
Conclusion The present study showed that climate and land suitability class in Neyriz area was suitable and relatively suitable for wheat and barley, respectively. Solute transport modeling showed that land suitability degree will decrease gradually and soil quality will decline over time by assuming constant irrigation and precipitation condition over the next 10 years. Therefore, preventing the expansion of soil salinity and degrading agricultural lands require serious considerations of the authorities in the crisis Managements