The natural environment of The Mekong Delta

Part Two: Updated data on climate
From data on monthly average climate parameters for 5 consecutive years (2015 - 2019) at 2 national hydro-meteorological stations in Mytho (My Tho) and Cantho (Can Tho) compiled by the Center for Hydrometeorological Information and Data of the General Department of Hydrometeorology - MONRE and other documents it may summarize the characteristics of climate in the Mekong Delta Region as follows.

2.1. Overview of the climate in the Mekong Delta Region
The Mekong Delta has a high and stable temperature throughout the year, mostly in the range of 25 - 27⁰C, the number of sunny hours is 2,200 - 2,400 hours per year, an average of 6 -7 hours per day; average evaporation volume is 1,000 - 1,300mm, relative humidity of the air is 78 - 82%. Average annual temperature: 27.5 - 27.8 ⁰C. The hottest time of the year is March, April, daytime temperature 28°C - 35°C; From June to February the temperature is the lowest but rarely below 20°C.
Statistics for the period 2000 - 2020 show that the average rainfall of the whole Mekong Delta is about 1,733mm (ranging from 1,400 to 2,200mm, heavy rainfall is concentrated in the Ca Mau peninsula from 1,900 to 2,200mm). The total amount of rainwater resources in the Mekong Delta is 65.4 billion m3/year. The total annual rainfall at some stations in the past 20 years has shown a tendency to decrease by 10 - 15 %.
In terms of space, the average annual rainfall is highest in the West Coast from 2,000 - 2,400mm (Camau, South Kiengiang), gradually decreasing in the East Coast: 1,700 - 1,800mm (Baclieu, Soctrang). The lowest in the area between the Hau (Bassac) and Tien (Mekong) rivers (Angiang, Tiengiang), slightly increased to the northeast with 1,600 - 1,800mm (Tanan, Mochoa).
Due to heavy rain in the whole basin from August to December, it often causes flooding for about 50% of the whole area. In contrast, in the dry season there is often a shortage of fresh water, especially in areas affected by saline intrusion.
2.2. Details of changes in climate features in the last 5 years
Details of the evolution of climate characteristics at two stations of Mytho in Tiengiang province (longitude: 106o24', latitude: 10⁰21' - representing the North of Hau river) and Cantho (longitude: 105⁰46', latitude degree: 10⁰02', representing the southern region of Hau River) for 5 consecutive years (2015 - 2019) according to data provided by the Center for Hydrometeorological Information and Data - General Department of Meteorology - MONRE (March, 2020)1 are outlined below.
2.2.1. Rain regime
The characteristics of total rainfall in the months of the year from 2015 - 2019 at Mytho and Cantho stations are shown in Figures 2.1 and 2.2

Figure 2.1. Left: Changes in monthly average rainfall and total annual rainfall at Mytho station (right).

Figure 2.2. Left: Changes in monthly average rainfall and total annual rainfall at Cantho station (right).

From the above tables and figures it can be commented:

- From 2016 - 2019, the average monthly rainfall and the total annual rainfall in Mytho decreased continuously.
- Unlike in Mytho, from 2016 - 2019, the average monthly rainfall and total annual rainfall in Cantho did not change much, except for a marked increase in 2017.
- The average rainfall for many years in the period 2015 - 2019: In My Tho, it changes quite clearly from 1201.5 - 1815.5mm; in Cantho from 1,498.2 - 2,088.4mm.
- The average monthly rainfall in the period 2015 - 2019 in Mytho changed quite clearly: From 100.1 - 151.3mm; in Cantho from 124.9 - 174mm.
- Thus, the rainfall in the area south of Hau river (represented by Cantho) is significantly higher than that in the North of Hau river (represented by Mytho).
- In Cantho and Mytho, the rainy season is from May to November and accounts for over 90% of the total annual rainfall.
- In the dry season: The monthly rainfall is usually less than 10mm, in the months of February and March, the rainfall = 0.0.
The average number of rainy days in many years at My Tho station is 127 - 156 days, at Cantho station is 119 - 181. September has the most average number of rainy days, February has the least average number of rainy days.
2.2.2. Atmospheric humidity
At Mytho and Cantho stations, the average annual relative humidity in the last 5 years fluctuated between 79 - 81% and did not change much between years.
The humidity in the rainy season (May - November) is usually in the range of 82 - 86% higher than the humidity in the dry season months (December - April, about 71 - 82%).

Figure 2.3. Average humidity (%) in 2015 - 2019: At Mytho station (left); at Cantho station (on the right).

2.2.3. Atmospheric temperature

The evolution of the annual average air temperature in MytTho and Cantho in the period 2015 - 2019 is shown in Figure 2.4.

Figure 2.4. Average annual temperature (oC), 2015 - 2019: At My Tho Station (left) and Can Tho Station (right).

From the above tables and figures it can be commented:
- At Mytho Station: The average temperature for many years is 27.6 - 27.8oC, April has the highest average temperature (29.0 - 29.6oC), January has the lowest average temperature ( 25.0 - 27.10oC). The absolute highest temperature is 37.0oC (April 2017) and the absolute lowest is 19.8C (January 2015).
- At Cantho Station: The average temperature for many years is 27.5 - 27.8oC (equivalent to My Tho), April has the highest average temperature (28.8 - 29.3oC, slightly lower than Mytho). little), January has the lowest average temperature (25.2 - 26.9oC). The absolute highest temperature is 36.7oC (April 2019) and the absolute lowest is 19.6oC (January 2015).
2.2.4. Evaporization
Evaporation at My Tho station is shown in Figure 2.5 below.
From the tables and figures above it can be seen that:
- The average annual evaporation at both stations is very high, with over 50% of the rainfall in some years. Especially, in the months of the dry season (12-4), especially in the months of February - April, evaporation is very high; combined with low humidity and high temperature creates unfavorable weather for agricultural production, daily life and human health.
- At My Tho station: The average annual evaporation is 875.7 - 1119.7mm; monthly average is 73 - 93.3mm.
- At Can Tho station: The annual average evaporation is 884.4 - 1,102.7mm; the monthly average is 73.7 - 91.9mm, equivalent to that at My Tho station.

Figure 2.5. Evaporation (mm) average monthly (left) and whole year (right) at My Tho Station, 2015 – 2019

Figure 2.6. Evaporation (mm) average monthly (left) and whole year (right) at Cantho Station, 2015 – 2019

2.2.6. Unusual weather phenomena
Big storms rarely happen in the Southern Vietnam. However, in the past decades, storms have tended to move to the south and every year, there are usually a number of storms approaching or hitting the Mekong Delta at the end of the year. One of the biggest natural disasters occurring in the Mekong Delta was Typhoon Linda (Typhoon No. 5) in 1997, causing serious damage along the coastal area from Tien Giang to Ca Mau: 3,000 people died and went missing, 200,000 houses were damaged. damage, material damage is estimated up to 385 million USD. In 2006, Typhoon Durian entered the Mekong Delta but caused less damage.

Compiled by VESDEC for Strategic Environmental Assessment (SEA) for the Mekong Delta Integrated Regional Planning, April 2020.
5 year data on climate in the Mekong Delta are avalable in VESDEC library