Adaptation strategies for rainfed rice water management under climate change in Songkhram River Basin, Thailand


 This study investigates the potential impacts of climate change on water resources and evaluates adaptation strategies on rainfed rice water management under climate change scenarios in the Songkhram River Basin, Thailand. The Soil and Water Assessment Tool (SWAT) model was used to project the future water availability under climate change scenarios for the period of 2020–2044. Future annual water availability is expected to remain unchanged due to unchanged future rainfall but expected to reduce from June to November due to changes in seasonal rainfall. The effects of supplying irrigation water to reduce the impact of climate change and increase rainfed rice production were evaluated. To increase the rice production by 15%, it is proposed to construct a reservoir with a capacity of below 65 MCM in each of the 15 sub-basins to fulfill the irrigation water requirements during the rainfed rice season. Alternatively, adaptation at the farm scale can be implemented by constructing ponds with a capacity of 900 m3 to store water for 1 ha of rice field to meet the potential rice yield during the non-rainfed rice season. The results of this study are helpful to policymakers in understanding the potential impacts of climate change and the formulation of adaptation strategies for water and rice sectors in the basin.


GRAPHICAL ABSTRACT INTRODUCTION
Climate change has negative impacts on crop growth and production throughout the world, including many areas in Thailand. Temperature is expected to rise, and higher variability in rainfall is expected at both global and local scales (IPCC ). Changes in magnitude and patterns of temperature and rainfall could significantly reduce rice production (Babel et al. ; Shrestha et al. ; Boonwichai et al. ). Climate change can be defined as a trend in one or more climatic variables characterized by a fairly smooth continuous increase or decrease of the average value during the long period of record including higher surface temperature, floods, droughts, storms, and sea-level rise.
The global mean surface temperature is expected to rise by 4.8 C in the 21st century (IPCC ). Thailand's temperature is expected to rise by approximately 2-3 C during the middle of the century and continue increasing until the end (Chinvanno & Center ).
Climate is one of the most important factors in agricultural productivity and could directly influence it since it is linked to physiological processes (Deb et al. ). This issue could affect global food security, especially in developing countries (Babel et al. ). Climate change may have both positive and negative impacts on the quantity and quality of agricultural productions, depending on location, climate zone, and crops (Deb et al. ). Studies have established the interdependence between climate and agriculture, and generally agree that actions have to be taken to ensure future water security and food security (Corkal et  Adaptation strategies can greatly reduce the magnitude of impacts on rice production under climate change conditions. Babel et al. ()

METHODOLOGY
The study was conducted following the methodological framework in Figure 1

Study area
The Songkhram River Basin is a sub-basin of the Mekong River Basin and the second-largest in Northeast Thailand  basin is agriculture, covering approximately 68% of the land area. There are two main rice-growing seasons: majorfrom June-November and secondaryfrom January-May. Rice yield in the basin depends mainly on rainfall, and the Office of Agricultural Economics in Thailand reported that the average major rice yield in the basin is 2.16 t/ha.

Meteorological data
Observed meteorological data (six temperature and eight rain gauge stations) were obtained from the TMD.

Land-use and land-cover data
Land-use and land-cover data for year 2013 were collected from the LDD (Figure 3(a)). The majority of land use in the basin is agriculture, covering about 8,800 km 2 or 68% of the total land area (Supplementary Material, Table S1).
Most of the area consists of paddy fields, rubber, and eucalyptus trees, taking up almost 93% of the agricultural land area in 2013. Water availability is the main issue in the basin due to water storage, as water body covers about only 4% of the basin area.

Soil data
Soil data for the SWAT model were obtained from the Har- where SW t is the final soil water content (mm), SW 0 is the initial soil water content on day i (mm), t is the time  Table S2).

RESULTS AND DISCUSSION
Climate change scenarios  Table 2). The minimum temperature is also expected to rise which varies between 0.6 and 1.4 C and between 0.6 and 1.6 C under RCP4.5 and RCP8.5 scenarios, respectively (Figure 4(b); Table 2). In contrast, future rainfall might increase or decrease depending on the location which varies between À0.5 and 14.5% and between À1.4 and 26.6% under RCP4.5 and RCP8.5 scenarios, respectively (Figure 4(c); Table 1).

Impacts of climate change on rainfed rice IWR
Temperature rise would increase both the CWR and IWR.
The IWR for KDML105 rice variety is projected to increase under climate change scenarios causing a significant  Table 5).

Sensitivity analysis
Sensitivity analysis focuses on the most influential parameters for reducing model uncertainty ( Figure S1 and Table S3).
Calibration, validation, and model performance evaluation The R 2 , RSR, NSE, and PBIAS were analyzed for the model performance evaluation. The model performance evaluation indicates satisfactory performance (Table 3).
The Since this study mainly focused on water availability than flooding, the performance is satisfactory.

Climate change impacts on water resources
The average annual discharge was approximately 12,631 MCM for the baseline period . The projected future annual discharge varied between 10,756 and 13,396 MCM and between 10,961 and 15,541 MCM under RCP4.5 and RCP8.5 scenarios, respectively, as shown in   and increase under the ICHEC-EC-EARTH-SMHI-RCA4 climate model under both scenarios (Figure 7; Table 4).
Reduced future water availability during the cropping season, especially the rice season, may affect crop growth and yield. Boonwichai et al. () reported that the water deficit has a significant impact on rice production.  V, the existing parameter value is to be replaced by a given value; R, an existing parameter value is multiplied by (1þ a given value).       Adaptation strategies for rainfed rice water management Adaptation strategies are necessary to cushion the impacts of climate change and increase crop production. The following subsections suggest adaptation strategies at both the river basin and farm scales.
Adaptation strategies for water management at the river basin scale  Table 5.
The future water availability could be enough to fulfill the IWR. The average annual future water availability is 11,803 and 12,445 MCM under RCP4.5 and RCP8.5 scenarios, respectively, for the period of 2020-2044. The future water availability of all 15 subbasins could vary between 51 and 1,746 MCM and between 51 and 1,953 MCM under RCP4.5 and RCP8.5 scenarios, respectively, as shown in Table 5 and Figure 8.
Proper water storage solutions, such as constructing a reservoir or a pond, are required for water management in the basin. However, a feasibility study should be conducted to determine the optimal location of the reservoir.
According to Rufin et al. (), water storage structures should be decentralized for allocation efficiency. To increase the rice production by 15%, it is proposed to construct a reservoir in each sub-basin to fulfill the IWRs. Foundation ). The land is divided into four parts with a ratio of 30% allocated for a pond to store rainwater, 30% for rice cultivation during the rainy season, 30% for fruit, vegetables, field crops, and herbs, and 10% for accommodation and other structures. This approach will be useful for reducing the impact of climate change in the future given cooperation from farmers and local stakeholders.

Implications and perspectives
In

CONCLUSIONS
In this study, the impact of climate change on water resources in the Songkhram River Basin was investigated using the SWAT model, and the effects of supplying irrigation water to reduce the impacts and increase rainfed rice production were evaluated at the river basin and farm scale. Results suggest that the uncertainty in future seasonal water availability would pose challenges for future water management. At the basin scale, the total future water availability is projected to be over 10,000 MCM under both RCP4.5 and RCP8.5 scenarios, enough to fulfill the IWR for all 15 sub-basins with proper water management techniques. Although the Government of Thailand just approved a 20-year (2018-2037) master plan for managing water resources, including a specific goal to achieve water security in the production sector, it is necessary to conduct location-specific studies to devise proper budget plans and strategies. Moreover, at the farm scale, adaptation strategies for water management during the rainfed rice season were proposed following government policies. In line with the government's efforts to enhance water security, the study suggests constructing reservoirs to store irrigation water as an adaptation strategy for water management at the farm scale. The ponds should measure 20 m in width, 15 m in length, and 3 m in depth to store 900 m 3 of water for each hectare of rice to overcome the water deficit and achieve the potential rice yield. Concerns such as maintenance, locations of pond constructions, and water network management to prevent and mitigate flood and drought issues, however, must be addressed with the participation of all stakeholders prior to implementation. This study thus serves as a preliminary step leading to the suitable policies, strategies, and actual action plan to ensure sustainability in the face of climate change.