Investigation of the effect of soil salinity and water quality on saffron daughter corms using crop modeling and measured data

Introduction

About 70% of the total freshwater withdrawal from resources is used in the agricultural sector. Water and soil salinity is one of the most important problems in the agricultural sector in arid and semi-arid regions. In such areas that are facing water shortages, saline water is commonly used in irrigated lands and it will be of great help in preserving freshwater resources. Crop modeling in combination with field measurements is an efficient method to improve water productivity in the field and investigate the crop's biological response to different field conditions. Several crop models have been developed for crop growth simulation. Among these models, AquaCrop software has been widely studied in recent years. AquaCrop software is able to simulate the growth process under different conditions with few input data that can be easily measured in the field. Few studies have been conducted on saffron crop modeling with AquaCrop software, but this model has not yet been calibrated to simulate salinity during the growing season. This research was carried out to calibrate the AquaCrop software for simulating the variations of soil salinity in the root zone of two-year-old saffron. In addition, in the present study, the effect of different levels of salinity and the application of different levels of organic mulch and their mutual effect on the yield of daughter corms, biomass, and water productivity were investigated.

Material and Methods

To calibrate the AquaCrop software for two-year saffron during the growing season in the research farm of the Ferdowsi University of Mashhad (FUM), growth parameters of saffron crop such as the soil moisture and salinity, the dry weight of daughter corms and the crop canopy cover were continuously measured during the growing season. Soil moisture and salinity were measured at least once a week, and the dry weight of daughter corms was measured biweekly. After model calibration, the accuracy of the model for simulating soil salinity during the growing season was evaluated by comparing the measured and simulated values. Statistical indicators of Pearson correlation coefficient, root mean square error, and Nash Sutcliffe model efficiency coefficient, were used to evaluate the accuracy of crop model simulation. The model was subsequently run for different initial conditions of soil salinity and irrigation water salinity. The dry weight of daughter corm, biomass, and ET water productivity were monitored for different conditions. Then, the model was run in the same conditions of water and soil salinity under the application of organic mulch, and the effect of mulch on the yield of daughter corms and evapotranspiration water productivity under salinity stress was investigated.

Results and Discussion

The statistical indicators between the measured and simulated values of soil moisture, canopy cover, and biomass approved the capability of AquaCrop for simulating saffron growth. Then, according to the measured salinity values of the root zone using the TDR sensor, AquaCrop was recalibrated to simulate soil salinity. Afterward, the changes in the measured salinity values of the root zone during the growing season were compared with the values simulated by AquaCrop. The Pearson correlation coefficient for measured and simulated soil salinity by software was 0.9 and the root mean square error was 0.086 dS m-1. Nash–Sutcliffe efficiency was 0.66, showing the high accuracy of AquaCrop for simulating soil salinity. The results of the crop growth simulation in saline conditions show the sensitivity of saffron to salinity. The results showed that under no initial salinity (0.5 dS m-1) in the soil, increasing the salinity of irrigation water from 1 dS m-1 to 4 dS m-1 caused a decrease of 3.7 % in the daughter corms weight. In addition, considering the initial salinity of 2 dS m-1, increasing the salinity of the irrigation water has a significant effect on reducing the daughter corm weight. In the presence of high-quality irrigation water (0.5 dS m-1), increasing the initial salinity of the soil from no salinity (0.5 dS m-1) to 4 dS m-1 caused a 38% decrease in the weight of daughter corms. The effect of organic mulch was also evaluated under saline water irrigation conditions. The results showed that the use of organic mulch with 100% coverage in water and soil salinity conditions equal to 4 dS m-1 can mitigate the effect of salinity stress by increasing 51% of daughter corm weight.

Conclusion

Water and soil salinity and its related problems are limiting factors in agricultural production in arid and semi-arid regions. The expansion of irrigation methods with saline water without proper management can lead to the risk of soil quality loss and in turn the loss of agricultural lands in the long term. In this research, the AquaCrop model was calibrated to simulate soil salinity during the saffron growing season, and the effect of organic mulch on soil and water salinity conditions was evaluated on the yield of daughter corms. The findings of this research will be of great help to farmers and water experts in improving the performance of saffron in saline soil and irrigation water.