Evaluation of the SALTMED model to simulate Potato yield and water productivity under different irrigation water management

Introduction

Potato is one of the most important food sources in the world and is considered one of the basic sources in different countries. The importance of this plant is so great that the Food and Agriculture Organization of the United Nations (FAO) has introduced it the fifth food crop in the world. The total area of potato cultivation in the world is about 17.34 million hectares. About 370 million tons of potatoes are harvested from these farms. According to published statistics, Iran is the 13th producer of this product with the production of about five million tons of potatoes. The yield of potato is high compared to other crops, and for this reason, it has a higher water efficiency than other crops. However, potato yield is strongly dependent on the amount of irrigation water. This issue has caused special attention to be paid to the amount of irrigation water and many studies have been conducted on determining the optimal irrigation water. However, providing the right amount of irrigation water in each climate and irrigation system requires many experiments. These tests require spending a lot of time and money, which cannot be done in research centers in the current situation. To solve this problem, various plant models have been presented. The SALTMED model is one of the plant models that is presented for plants, soils, different irrigation systems, irrigation management solutions, different water qualities and the effect of environmental stresses.



Materials and Methods

To carry out this research, the data collected from two research projects were conducted (the first project in 2012 and 2013, the second project in 2017 and 2018) in Shahrekord's research station. In the research projects, to manage irrigation in potato cultivation, two irrigation factors were investigated: the first factor includes different irrigation methods (S: drip, Su: subsurface drip and F: furrow) and quantitative management of irrigation water as the second factor (FI: providing 100% of water needs, RDI80: providing 80% of water needs and RDI65: providing 65% of water needs) considering three repetitions. Data collected from the first year were used to calibrate the SALTMED model. To evaluate the SALTMED model at this stage, the statistics of root mean square error (RMSE), normalized root mean square error (NRMSE), mean bias error (MBE), model efficiency (EF), agreement index (d) and determination of coefficient (R2) were used. Validation of this model was done using second-year data.



Results and Discussion

The highest and lowest difference between observed and simulated values was 4.2 and 1.4 tons per hectare, respectively. The average difference between observed and simulated values was equal to 2.7 tons per hectare. The difference between observed and simulated performance for drip, subsurface drip and furrow irrigation methods was equal to 3, 1.9 and 3.1 tons per hectare, respectively. Also, the accuracy of this model for simulating yield in the furrow irrigation method was lower than the other two methods. The highest and lowest difference between observed and simulated water productivity was 0.3 and 1.3 kg.m-3, respectively. The average of this difference was determined to be 0.7 kg.m-3. The difference between observed and simulated water productivity for drip irrigation, subsurface drip and furrow irrigation methods was 0.8, 0.5 and 0.8, respectively. Based on the values of MBE statistics, the SALTMED model had an underestimation error in simulating yield and water productivity. The results of the NRMSE statistic showed that the accuracy of the SALTMED model for yield simulation was in the excellent category. The efficiency of the SALTMED model for yield simulation was acceptable based on two statistics EF and d. The R2 statistic for the simulation of potato yield by this plant model varied between 0.88-0.99. Using the recalibrated the SALTMED model, a simulation of yield and water productivity was done to meet the water needs of 90, 55 and 45% of potatoes for all three irrigation methods. A downward trend was observed between the reduction of water supply and potato yield in all three irrigation methods. The yield difference between 100% and 90% and 90% and 80% water supply in surface irrigation was 4.8% and 5.7%, respectively. These values were equal to 18.6 and 8.7 percent for drip irrigation and 13.3 and 1.7 percent respectively for subsurface drip irrigation. Therefore, the slope of yield reduction until providing 80% of the water requirement was low in all three irrigation methods.



Conclusion

This research was conducted to evaluate the SALTMED model for simulating the yield and water productivity of potatoes under three irrigation methods: furrow, drip and subsurface drip. The results showed that the SALTMED model had an underestimation error for simulating both parameters (MBE<0). However, the obtained error is negligible and the accuracy of this plant model was in the excellent category (NRMSE<0.1). Based on the results of EF (<0.88) and d (<0.99) statistics, the effectiveness of the SALTMED model was favourable for simulating both yield and water productivity. To determine the optimal amount of irrigation water, the recalibrated SALTMED model was used. The results showed that providing 80% of the potato's water requirement led to the achievement of optimal yield and high water productivity.