Identification of different drought patterns of dryland wheat in the northwest of Iran by APSIM model

Introduction:

Improving crop yields can meet the projected demand in developed and developing countries as a key and promising solution. To increase dryland wheat production in arid and semi-arid regions such as Iran, researchers and crop breeders need to understand the drought pattern (in terms of time and intensity) occurring in wheat agro-systems because depending on the time and intensity of drought stress, drought stress impacts on different processes and genes of crops. To accurately study the drought pattern of dryland wheat and different cultivars adapted to these conditions, researchers need to design multi-environmental field experiments, which are time-consuming and costly. Under these circumstances, some essential information is limited in the field experiments, especially when done for only a few years, and basically does not indicate fluctuations in the target environments. In contrast, the modeling and simulation approach is a useful method for comprehensive environmental impact assessment. 

Materials and Methods:

This study was conducted to identify and study different drought patterns in 8 locations including Ardebil, Khalkhal, Maragheh, Marand, Meshginshahr, Sarab, Tabriz, and Urmia in the northwest of Iran. APSIM model was used to simulate the growth and development of dryland wheat in the study locations. The long-term climatic included minimum and maximum temperatures, rainfall, and radiation (1980 to 2016) collected from the Meteorological Organization of Iran. These data were used as input to the crop growth simulation model. Water supply and demand index was used to determine different drought patterns. This index is obtained by dividing soil water content to plant water demand (WSDR). After calculating the water supply and demand index for each year and region, the CLARA clustering method was used to classify and group the index. The number of groups obtained by CLARA method was equal to the type of drought patterns of dryland wheat. 

Results and Discussion:

The results showed 4 different drought patterns during the dryland wheat growing season in the study locations.  Drought pattern 1 showed no drought stress or mild drought stress throughout the growing season of dryland wheat. Under drought pattern 2, drought stress started before flowering and ended during seed filling. For drought pattern 3, water stress started from the time of germination but removed during the vegetative growth period. Drought pattern 4 started in the early stages of growth and continued until the end of the dryland wheat growing season. The frequency of occurrence of 1, 2, 3, and 4 drought patterns was 27.8%, 26%, 8% and 38.2%, respectively. On average across locations, transpiration was 160, 137.1, 31, and 86.7 mm for 1, 2, 3, and 4 drought patterns during the wheat growing season. Also, a significant difference was observed among different drought patterns in terms of biomass that biomass under drought patterns 1, 2, 3, and 4 was equal to 13411, 10076, 2097 and, 6435 kg ha-1, respectively. Grain yield under drought pattern types 1, 2, 3 and 4 were 3738.7, 2949.7, 694.6, and 1456.2 kg ha-1, respectively. The highest percentage of light and mild drought patterns (1 and 2) was related to Marand region with 100% and Sarab region experienced more severe drought patterns (3 and 4) than other regions (88.2%). Under different locations and drought patterns, on average across years, the highest grain yield was simulated for Marand under drought pattern 1 (4759 kg ha-1) and the lowest was recorded for Urmia under drought pattern 3 (208.8 kg ha-1). 

Conclusion:

In general, the simulation results showed that 4 different types of drought patterns were observed during the dryland wheat growing period in the study locations. The frequency of occurrence of 4 drought patterns was various in the study locations and the grain yield varied under the drought patterns. However, the highest occurrence was related to drought pattern 4, under which drought stress begun in the early stages of growth and lasted until the end of the dryland wheat growing season. Accordingly, it is suggested to consider various strategies such as using cultivars with short maturity period to avoid drought stress at the end of the season and using cultivars with high resistance to drought stress (especially in the flowering period).