Evaluation of current meteorological and hydrological drought and its future forecast in the Tajan watershed

Abstract

Introduction

Climate, due to its influential role in other natural hazards such as drought, can affect the economy and people's lives, especially in the production of different sectors of agriculture, animal husbandry and industry, and cause damage and destruction in these sectors in different regions. Drought is one of the biggest climatic challenges that our country is facing in recent years (Mofidipour, 2016). Climatic changes cause many problems due to the effect it has on the temporal and spatial changes of precipitation in different regions. Due to its gradual process and slow speed, this phenomenon has been operating for a relatively longer period of time and its effects may be revealed after a few years and with a longer delay than other risks (Mozaffari et al., 2014; Saedi et al., 2018). This phenomenon creates more tangible effects in rural areas due to the greater vulnerability of rural communities. Therefore, the knowledge of this phenomenon and the analysis and evaluation of its results on the existing ecosystems in a watershed as a planning and decision-making unit are necessary and necessary for the adoption of suitable adaptation strategies (Mozaffari et al., 2022; Ding et al. 2021).

Material and Methods

To conduct this research, after collecting the data, the SPI index was used to evaluate the meteorological drought. Also, to check the trend of flow rate changes, the data of existing hydrometric stations in the region, which have more complete data, were used. Lars-WG software was used to predict the future climate under two optimistic (RCP2.6) and pessimistic (RCP8.5) climate scenarios. After preparing the current and future data, DrinC software was used to determine the drought thresholds based on the SPI index standard table. In this research, the IHACRES hydrological model was also used to predict discharge in the coming period. For this purpose, using the long-term data of future precipitation and temperature prepared from the previous stage, as well as the observed discharge, the trend of the discharge data in the future was predicted, then using the SDI method in the DrinC software, the hydrological drought of the Tajan watershed was calculated at the outlet station (Kordkhil). After analyzing the stream flow data, using the standard table related to the SDI method, the thresholds of hydrological drought were determined.

Results and Discussion

The future climate was predicted for a period of about 27 years (2023-2050) under the influence of two optimistic scenarios (RCP2.6) and pessimistic scenario (RCP8.5) in three selected stations (Kordkhil, Soleiman Tange, Rig Cheshme). The results of the climatic drought situation and the index values of the predicted period (2023-2050) of Soleiman Tange station in 12-month steps showed that it can be expected that in the water year 2050-2049, the possibility of a very severe climatic drought may occur in this region. Also, the results show that there is no significant difference in the drought index of the two scenarios in the future.

The results of the climate drought situation of Rig Cheshme station in 12-month time steps showed that in the water year 2029-2030 there is a possibility of a very severe climate drought in this region. Also, the results of the climatic drought condition of the Kordakhil station in 12-month time steps showed that the condition of the Kordakhil station is somewhat different compared to the other two stations that are located at a higher altitude. This difference is probably due to the proximity of this station to the Caspian Sea and the climate of that region is affected by the humidity of the coast. It can also be expected that there will be a very severe climatic drought in the water year 2032-2033 in this region.

The results of the IHACRES model showed that the comparison chart of the simulated discharge and the observed discharge are in good agreement with each other. Also, the value of the error coefficient in the two stages of evaluation and recalibration has been obtained as 0.48 and 0.53, respectively, which indicates the acceptable ability of the model in simulating the future discharge. The results of studying the hydrological drought trend of Kordakhil station using SDI index were done in a period of 20 years and in time steps of 3, 6 and 12 months. The results show that the discharge rate of the watershed will not change much in the future due to climate change, and the reason is probably heavy rainfall events due to climate change, which causes the river discharge to flood. Considering that these results only show the effect of climate change on discharge, there is a possibility that due to other issues such as land use changes, the amount of discharge will undergo other changes.

Conclusion

The results of this research show that in both climate scenarios, there is a possibility of even very severe droughts in some years in the future, although in some years the climate can be very wet. These changes in the state of drought in a period of about 30 years indicate climate fluctuations, which is one of the signs of climate change in a region. Therefore, based on the results obtained, it cannot be said that in the next 30 years, we will be in a completely dry state or in a completely wet state. Therefore, it is necessary to use adaptation approaches to reduce the effects of drought in some years. The analysis of the drought condition of Tajen River also shows fluctuations, so that in some years the trend of flow is decreasing and in some years it is increasing however, relying on the study of the flow rate of the region alone cannot accurately indicate the drought situation because some measures in watersheds can greatly affect the flow rate of a river and cause changes in the flow rate data. Based on this, all actions and approaches adopted in the watershed areas should be based on adaptation to climatic fluctuations to avoid severe damages in times of drought (flash floods) and drought (agricultural losses).