Mou Leong Tan

Evaluation of six high-resolution satellite and ground-based precipitation products over Malaysia

Satellite precipitation products (SPPs) potentially constitute an alternative to sparse rain gauge networks for assessing the spatial distribution of precipitation. However, applications of these products are still limited due to the lack of robust quality assessment. This study compares daily, monthly, seasonal, and annual rainfall amount at 342 rain gauges over Malaysia to estimations using five SPPs (3B42RT, 3B42V7, GPCP-1DD, PERSIANN-CDR, and CMORPH) and a ground-based precipitation product (APHRODITE). The performance of the precipitation products was evaluated from 2003 to 2007 using continuous (RMSE, R2, ME, MAE, and RB) and categorical (ACC, POD, FAR, CSI, and HSS) statistical approaches. Overall, 3B42V7 and APHRODITE performed the best, while the worst performance was shown by GPCP-1DD. 3B42RT, 3B42V7, and PERSIANN-CDR slightly overestimated observed precipitation by 2%, 4.7%, and 2.1%, respectively. By contrast, APHRODITE and CMORPH significantly underestimated precipitations by 19.7% and 13.2%, respectively, whereas GPCP-1DD only slightly underestimated by 2.8%. All six precipitation products performed better in the northeast monsoon than in the southwest monsoon. The better performances occurred in eastern and southern Peninsular Malaysia and in the north of East Malaysia, which receives higher rainfall during the northeast monsoon, whereas poor performances occurred in the western and dryer Peninsular Malaysia. All precipitation products underestimated the no/tiny (<1 mm/day) and extreme (≥20 mm/day) rainfall events, while they overestimated low (1–20 mm/day) rainfall events. 3B42RT and 3B42V7 showed the best ability to detect precipitation amounts with the highest HSS value (0.36). Precipitations during flood events such as those which occurred in late 2006 and early 2007 were estimated the best by 3B42RT and 3B42V7, as shown by an R2 value ranging from 0.49 to 0.88 and 0.52 to 0.86, respectively. These results on SPPs’ uncertainties and their potential controls might allow sensor and algorithm developers to deliver better products for improved rainfall estimation and thus improved water management.

Impacts of land-use and climate variability on hydrological components in the Johor River basin, Malaysia

Abstract This study aims to investigate separate and combined impacts of land-use and climate variability on hydrological components in the Johor River Basin (JRB), Malaysia. The Mann-Kendall and Sen’s slope tests were applied to detect the trends in precipitation, temperature and streamflow of the JRB. The Soil and Water Assessment Tool (SWAT) was calibrated and validated using measured monthly streamflow data. The validation results showed that SWAT was reliable in the tropical JRB. The trend analysis showed that there was an insignificant increasing trend for streamflow, whereas significant increasing trends for precipitation and temperature were found. The combined (climate + land-use change) impact caused the annual streamflow and evaporation to increase by 4.4% and 1.2%, respectively. Climate (land-use) raised annual streamflow by 4.4% (0.06%) and evaporation by 2.2% (−0.2%). Climate change imposed a stronger impact than land-use change on the streamflow and evaporation. These findings are useful for decision makers to develop better water and land-use policies. Editor Z. W. Kundzewicz; Guest editor V. Krysanova

Assessment of GPM and TRMM Precipitation Products over Singapore

The evaluation of satellite precipitation products (SPPs) at regional and local scales is essential in improving satellite-based algorithms and sensors, as well as in providing valuable guidance when choosing alternative precipitation data for the local community. The Tropical Rainfall Measuring Mission (TRMM) has made significant contributions to the development of various SPPs since its launch in 1997. The Global Precipitation Measurement (GPM) mission launched in 2014 and is expected to continue the success of TRMM. During the transition from the TRMM era to the GPM era, it is necessary to assess GPM products and make comparisons with TRMM products in different regions to achieve a global view of the performance of GPM products. To this end, this study aims to assess the capability of the latest Integrated Multi-satellite Retrievals for GPM (IMERG) and two TRMM Multisatellite Precipitation Analysis (TMPA) products (TMPA 3B42 and TMPA 3B42RT) in estimating precipitation over Singapore that represents a typical tropical region. The evaluation was conducted at daily, monthly, seasonal and annual scales from 1 April 2014 to 31 January 2016. The capability of SPPs in detecting rainy/non-rainy days and different precipitation classes was also evaluated. The findings showed that: (1) all SPPs correlated well with measurements from gauges at the monthly scale, but moderately at the daily scale; (2) SPPs performed better in the northeast monsoon season (1 December–15 March) than in the inter-monsoon 1 (16 March–31 May), southwest monsoon (1 June–30 September) and inter-monsoon 2 (1 October–30 November) seasons; (3) IMERG had better performance in the characterization of spatial precipitation variability and precipitation detection capability compared to the TMPA products; (4) for the daily precipitation estimates, IMERG had the lowest systematic bias, followed by 3B42 and 3B42RT; and (5) most of the SPPs overestimated moderate precipitation events (1–20 mm/day), while underestimating light (0.1–1 mm/day) and heavy (>20 mm/day) precipitation events. Overall, IMERG is superior but with only slight improvement compared to the TMPA products over Singapore. This study is one of the earliest assessments of IMERG and a comparison of it with TMPA products in Singapore. Our findings were compared with existing studies conducted in other regions, and some limitations of the IMERG and TMPA products in this tropical region were identified and discussed. This study provides an added value to the understanding of the global performance of the IMERG product.

Evaluation of TMPA 3B43 and NCEP-CFSR precipitation products in drought monitoring over Singapore

ABSTRACT Satellite and reanalysis precipitation products can be potentially used for monitoring and analysing drought patterns in tropical regions. In this study, Tropical Measuring Mission Multisatellite Precipitation Analysis monthly product (3B43) and the product from National Centers for Environmental Prediction Climate Forecast System Reanalysis (NCEP-CFSR) are analysed for precipitation estimation and drought assessment over Singapore. The evaluation is conducted by comparing the field observations from 22 precipitation gauges from 1998 to 2014. The Standardized Precipitation Index recommended by the World Meteorological Organization was used for the drought assessment. Results show that 3B43 performed better in monthly, seasonal, and annual precipitation estimations compared to NCEP-CFSR. Both products show an underestimation of monthly and annual precipitation. Similarly, 3B43 also outperformed NCEP-CFSR in drought monitoring over Singapore. The 3B43 product successfully detected major droughts that occurred in 2005, 2009, and 2014. Nevertheless, 3B43 reported a higher drought intensity compared to gauge measurements in recent drought events. Application of the NCEP-CFSR in drought monitoring is not recommended due to the low correlations with gauge measurements. In addition, NCEP-CFSR is unable to detect the major drought events correctly, except for the 2009 drought. This study serves as a reference for satellite algorithm and drought index designers to develop better products in the future.

Hydro-Meteorological Assessment of Three GPM Satellite Precipitation Products in the Kelantan River Basin, Malaysia

The rapid development of Satellite Precipitation Products (SPPs) has heightened the need for a hydro-meteorological assessment of the Integrated Multi-satellite Retrievals for Global Precipitation Measurement (IMERG) products in different climate and geographical regions. Reliability of the IMERG early (IMERG_E), late (IMERG_L) and final (IMERG_F) run products in precipitation estimations was evaluated over the Kelantan River Basin, Malaysia from 12 March 2014 to 31 December 2016. The three IMERG products were then incorporated into a calibrated Soil and Water Assessment Tool (SWAT) model to assess their reliability in streamflow simulations. Overall, monthly precipitation variability is well captured by the three SPPs. The IMERG_F exhibited a smaller systematic bias (RB = 7.14%) compared to the IMERG_E (RB = −10.42%) and IMERG_L (RB = −17.92%) in daily precipitation measurement. All the three SPPs (NSE = 0.66~0.71 and R2 = 0.73~0.75) performed comparably well as precipitation gauges (NSE = 0.74 and R2 = 0.79) in the daily streamflow simulation. However, the IMERG_E and IMERG_L showed a significant underestimation of daily streamflow by 27.6% and 36.3%, respectively. The IMERG_E and IMERG_F performed satisfactory in streamflow simulation during the 2014–2015 flood period, with NSE and R2 values of 0.5~0.51 and 0.62~0.65, respectively. With a better peak flow capture ability, the IMERG_F outperformed the near real-time products in cumulative streamflow measurement. The study has also shown that the point-to-pixel or pixel-to-pixel comparison schemes gave comparable conclusions. Future work should focus on the development of a standardized GPM hydro-meteorological assessment framework, so that a fair comparison among IMERG validation studies can be conducted.

Integrating manual calibration and auto-calibration of SWAT model in Muar Watershed, Johor

The Soil and Water Assessment Tool (SWAT) is a freely accessible hydrological modeling tool designed to simulate the impact of land-cover changes on hydrological processes over a continuous period. To test its performance in tropical climate conditions, the hydrological processes of the upper Muar watershed (UMW), Malaysia was modeled for land cover data in the year 1984. The SWAT model was run using 42 years of daily climate data from 1970 to 2011, with a six-year warm-up period (1970 to 1975). Manual calibration and auto-calibration using Sequential Uncertainty Fitting (SUFI-2) were coupled to optimize the Nash-Sutcliffe efficiency (NSE) and coefficient of determination (R2) value. Parameter CN2, CANMX, SOLAWC, GWREVAP and GWQMN are five of the most sensitive parameters for UMW. Meanwhile, EPCO, GWDELAY and CHN(2), found as less sensitive parameters to be adjusted. The value of NSEcalibration-validation = 0.71 & 0.62, and R2calibration-validation = 0.74 & 0.68 indicate of satisfied results obtained. The P-factor and R-factor for calibration was 0.92 and 1.77 respectively and P-factor=0.94, R-factor = 2.04 during validation period. This study therefore suggests that manual and auto-calibration approaches are suitable to be integrated to optimize NSE and R2 values. These goodness-of-fit statistics, supported by graphical representations, showed that the SWAT model could simulate a tropical watershed with reasonable accuracy.

Extreme Precipitation and Floods: Monitoring, Modelling, and Forecasting

1Chair of Hydrology and River Basin Management, Technical University of Munich, Arcisstrasse 21, 80333 Munich, Germany 2Guangdong Engineering Technology Research Center of Water Security Regulation and Control for Southern China, Sun Yat-sen University, Guangzhou 510275, China 3Julie Ann Wrigley Global Institute of Sustainability, Arizona State University, Tempe, AZ 85287, USA 4Geography Section, School of Humanities, Universiti Sains Malaysia, 11800 Penang, Malaysia