## 2 Materials and Methods
### Rain Gauges  
Daily precipitation from 38 principal rain gauges from the Malaysia Meteorological Department (MMD) was selected for this study because of the continuity of the recording from 2003 to 2007 and high confidence in the data [25] (Figure 1 and Table 1). These specific rain gauges are calibrated every six months and any breakdown is fixed within 7 days. Previous investigations showed the error for daily precipitation is below 5% [22]. In addition, 304 additional rain gauges from the Department of Irrigation and Drainage (DID) Malaysia with no missing data from 2003 to 2007 were considered. The daily rainfall values for MMD and DID rain gauges were taken at 08:00 a.m. local time [26], which is the same as 00:00 UTC time. Since most satellite precipitation products estimated daily precipitation at 00:00 UTC, the observed daily rainfall values collected from rain gauges were directly compared to the precipitation products.  
\begin{table}
\begin{tabular}{c c c c c} \hline
**No** & **ID** & **Station Name** & **Longitude (\({}^{\circ}\))** & **Latitude (\({}^{\circ}\))** & **Height (m)** \\ \hline \multicolumn{5}{c}{**Zone A (Northern Peninsula Malaysia)**} \\ \hline
1 & 41529 & Perai & 100.40 & 5.35 & 1.5 \\
2 & 48600 & Pulau Langkawi & 99.73 & 6.33 & 6.4 \\
3 & 48601 & Bayan Lepas & 100.27 & 5.30 & 2.8 \\
4 & 48602 & Butterworth & 100.40 & 5.47 & 2.8 \\
5 & 48603 & Alor Setar & 100.40 & 6.20 & 3.9 \\
6 & 48604 & Chuping & 100.27 & 6.48 & 21.7 \\ \hline \multicolumn{5}{c}{**Zone B (Eastern Peninsula Malaysia)**} \\ \hline
7 & 48615 & Kota Bharu & 102.28 & 6.17 & 4.6 \\
8 & 48616 & Kuala Krai & 102.20 & 5.53 & 68.3 \\
9 & 48618 & K. Terengganu Airport & 103.10 & 5.38 & 5.2 \\
10 & 48619 & Kajiklim K. Terengganu & 103.13 & 5.33 & 35.1 \\
11 & 48657 & Kuantan & 103.22 & 3.78 & 15.3 \\ \hline \multicolumn{5}{c}{**Zone C (Middle Peninsula Malaysia)**} \\ \hline
12 & 48631 & K.Tanah Ratah & 101.38 & 4.47 & 1471.6 \\
13 & 48632 & Cameron Highlands & 101.37 & 4.47 & 1545.0 \\
14 & 48642 & Batu Embun & 102.35 & 3.97 & 59.5 \\
15 & 48649 & Muadzam Shah & 103.08 & 3.05 & 33.3 \\
16 & 48653 & Temerloh & 102.38 & 3.47 & 39.1 \\ \hline \multicolumn{5}{c}{**Zone D (Western Peninsula Malaysia)**} \\ \hline
17 & 48307 & Universiti Malaya & 101.65 & 3.12 & 104.0 \\
18 & 48620 & Sitiawan & 100.70 & 4.22 & 7.0 \\
19 & 48623 & Lubok Merbau & 100.90 & 4.80 & 77.5 \\
20 & 48625 & Ipoh & 101.10 & 4.57 & 40.1 \\
21 & 48647 & Subang & 101.55 & 3.12 & 16.5 \\ \hline \end{tabular}
\end{table}
Table 1: Thirty-eight principal rain gauges across Malaysia, from the Malaysia Meteorological Department.  
Since the TRMM 3B42V7 used the Global Precipitation Climatology Centre (GPCC) gauge-based gridded monthly precipitation to perform the month-to-month gauge adjustment [27] and the APHRODITE applied the MMD rain gauges to produce the rainfall product [28], comparison of both products with the MMD observations inevitably leads to a better performance compared to other precipitation products. Rain gauge information such as the number and locations of stations applied in the GPCC production is not available to the public, according to international agreement [29]. However, the DID rain gauges were not used to construct the APHRODITE and Malaysia only supplied 37 rain gauges' monthly precipitation for the GPCC product development [30; 31; 32]. Therefore, these observations (342 rain gauges) can be considered as independent from gauge-adjusted precipitation products.  
For evaluation, the daily precipitation values of the pixels where rain gauges are located were extracted from various precipitation products and then aggregated to monthly and annual precipitation values. The mean annual precipitation (MAP) of each rain gauge was interpolated using the inverse distance weighting (IDW) technique with a 0.01\({}^{\circ}\) grid mesh for visualization purposes (Figure 2). This technique was selected as it is widely used because of its simplicity and reliability [33; 34].