Water Balance

INTRODUCTION

Global warming causes dramatic changes in water balance components such as precipitation, evapotranspiration, streamflow and baseflow, which could deplete affect freshwater supply during the dry season and exacerbate flood levels during the wet season. Malaysian government has recently agreed to the Paris Agreement’s target of limiting global temperatures to well below 2.0°C and pursue efforts to keep them below 1.5°C. This action is expected to reduce the risk and impacts of climate change on water balance significantly. Unfortunately, the impact assessment of global warming 1.5°C and 2.0°C on the water balance, which is required for policy, adaptation and mitigation strategies formulations has not been properly studied and discussed around the world, including in Malaysia. Therefore, this project aims (1) to investigate the capability of SWAT+ in extreme flow simulations in the Kelantan and Muda River Basins; (2) to evaluate various climate model bias-correction schemes for river basin scale modelling; (3) to simulate the spatial and temporal changes of hydro-climatic at basin scale under the 1.5°C and 2.0°C global warming; and (4) To develop an integrated water hazard risk assessment matrix in the selected river basins. We will incorporate the CORDEX-SEA CMIP5 and the latest CMIP6 climate projections (Project 1) into a calibrated SWAT+ model (this project) to project future water balance of both the basins. The projected water balance components will be converted to water stress and flood indices for each sub-basin. The maps will then be combined with other related geospatial data (i.e. population, river, transportation, river, etc.) to generate Integrated Water Disaster Risk (IWDR) maps under various climate scenarios. The findings and IWDR maps will be disseminated to other projects for risk system development (Project 1), water-related health (Project 3) and social-economic (Project 4) impact assessment.

OBJECTIVES

To investigate the capability of SWAT+ in extreme flow simulations in the Kelantan and Muda River Basins.
To evaluate various climate model bias-correction schemes for river basin scale modelling.
To simulate the spatial and temporal changes of hydro-climatic at basin scale under the 1.5°C and 2.0°C global warming.
To develop an integrated water hazard risk assessment matrix in the selected river basins.

METHODOLOGY

In general, this study will be conducted using a combination of the CORDEX Southeast Asia (CORDEX-SEA) domain climate projections with the elements of global warming 1.5°C and 2.0°C and SWAT+ that was just recently released by the SWAT developers. Besides that, future land use will be collected from PlanMalaysia for modelling, so that the impact of land use changes on water balance components such as streamflow, evapotranspiration and surface runoff can be captured more accurately. The methodology of this study will be divided into five major phases based on the objectives.

Phase 1: SWAT+ Setup, Calibration and Validation

SWAT+ is a completely restructured and enhanced version of SWAT with additional capabilities (Bieger et al. 2017). As mentioned earlier, this is the first study to evaluate the capability of SWAT+ in Malaysia. SWAT+ setup for KRB and MRB will begin with the updating of data collection such as the recent Digital Elevation Model (DEM), land use, soil, observed rainfall, temperature and streamflow data. Recent and future land use maps will be collected from the Department of Agricultural and PlanMalaysia. Then, sensitivity analysis will be conducted to identify the most sensitive parameters for SWAT+ modelling. Observed streamflow data will be used for calibration and validation of SWAT+. When the performance of SWAT+ reach to a certain level, it will then be used for the water balance analysis.

Phase 2: Bias correction of CORDEX-SEA projections.

Since the high-resolution (5 km) regional climate downscaling simulations of CMIP6 models over Peninsular Malaysia will be conducted by project 1, the second phase of this project will focus only on the bias correction of climate projected that generated by project 1. Several bias correction methods such as quantile mapping and delta approach will be tested to check their reliability in the bias correction. Identifying an appropriate bias correction approach is important to design a climate model bias-correction scheme for river basin scale modelling.

Phase 3: Incorporation of bias corrected CMIP6 climate projections into calibrated SWAT+

The third phase will involve the incorporation of the bias corrected CMIP6 climate projections (phase 2) and future land use map into calibrated SWAT+ (phase 1) to project future water balance within KRB and MRB. Normally, the simulation periods will be divided into baseline, near-, mid- and late-21st century. The future simulated water balance under three time slices will be compared with the baseline period to obtain the differences of water balance under global warming 1.5°C and 2.0°C.

Phase 4: Assessment of water balance

In the fourth phase, the potential water balance changes under global warming 1.5°C and 2.0°C will be evaluated by comparing the values between the future and historical periods. The assessment will be based on the Representative Concentration Pathways (RCPs) and Shared Socioeconomic Pathways (SSPs) scenarios. Major water balance components such as streamflow, evapotranspiration, runoff, baseflow, soil moisture, water yield, etc., will be obtained from SWAT. Then, standardized streamflow index (SSI) will be measured using the projected monthly streamflow to study the potential flood or dry spells in terms of intensity, duration and frequency.

Detailed map and location of Muda River Basin

Detailed map and location of Kelantan River basin

Flow chart of Project 2 depicting the connectivity among different phases of the Project

Phase 5: Integrated Water Disaster Risk mapping

The final phase involves the development of an integrated water hazard risk matrix that is suitable for KRB and MRB. It will be useful to understand the potential of water stress and flood risks under global warming 1.5°C and 2.0°C in KRB and MRB. For example, SWAT-simulated streamflow data from each sub-basin will be used to measure SSI, as one of the inputs to calculate the water hazard risk. Other geospatial data such as land use, population, river, transportation, slope, elevation, soil and other points of interest will be collected and manipulated using the Geographical Information System (GIS). The developed integrated water hazard risk mapping will be beneficial in identification of locations with low, moderate and high water hazard risks. Negative values could be used to indicate water stress, meanwhile positive values as flood. Then, the sub-basin layer will overlap with the land use map to calculate the area of high risk zones and the corresponding land use types. This information will then be passed to Project 1 for the risk management system development, Project 3 for the health impact assessment, and Project 4 for studying the social-economic impact under global warming 1.5°C and 2.0°C. The outputs will be very useful for the formulation of climate adaptation and mitigation strategies.