Depart. Of Meteorological Services Agro-met Office P.O. Box 10100, Gaborone Tel: 3612200 Fax: 3956282/140 Corner Maaloso- Metsimothaba Road Gaborone Village Highlights: Very wet to extremely wet conditions were experienced countrywide during the first dekad of January 2017. Cooler temperatures and cloudy conditions prevailed for more than two weeks. Reduction In rainfall to be expected over the entire country for the week of 17 th till 22 nd. Eastern parts to experience little to no rainfall for the next two weeks, 17 th till the 2 nd of February 2017. Inside this issue: 1. Weather Summary (2) 2. Drought Monitoring (3) 3. Farmer Tips (4) 4. Weather and Climate Outlook (4) 5. Impacts & Summary (5) Volume 2: Issue 3 BOTSWANA AGROMETEOROLOGICAL MONTHLY BULLETIN Background information on Botswana Botswana has rainfall pattern that is highly influenced by the position of the Inter Tropical Convergence Zone (ITCZ). The movement and intensity of the ITCZ determines the intensity of the seasonal rainfall, particularly during the second half of the rainy season (January March). Botswana is also indirectly influenced by the position of tropical cyclones in the West Indian Ocean and the passage of easterly waves; dry conditions are occasionally experienced in association with such weather systems. The mid-level westerly waves may create an unstable environment conducive for rainfall. The strength of Botswana sub-tropical high pressure belt determines the low-level influx of tropical moisture into the country. During summer (October to February), extremely high temperatures are experienced over the country with frequent dry spells and heat waves. Botswana is highly dependent on the rain-fed subsistence agriculture, and its rainfall pattern is very erratic, hence continuous monitoring and early warning is vital for maximizing agricultural produce. The intention of the agro-met bulletin is to encourage farmers to use weather and climate information in order to increase their crop yields. Other stakeholders can also make use of the information in the bulletin for planning and mitigation measures whenever there is need. In order for the bulletins to be useful to as many farmers and other users as possible, the weather and climate information from all four quadrants of the country are presented. Fig 1: Long Term Average Rainfall for the Month Of December DECEMBER 2016 Fig 1 Shows the monthly long term average rainfall for the month of December. Normally for the month of December, the north-eastern parts show an average rainfall of between 90mm and 130mm over the Chobe enclave. The south western parts on average will receive between 30mm and 60mm of rainfall. Whereas the south-eastern parts get between 60mm and 100mm. Page 1 of 6
Terms and Meaning: Dekad: 10 day rainfall measuring period Normal Rainfall: Between - 20% and 20 % of the average Below Normal Rainfall: Rainfall between -21% to -30% of the average Above Normal Rainfall: Rainfall between 20% and 30% of the average Much below Normal Rainfall: Rainfall below -30% of the average Much above Normal Rainfall: Rainfall above 30% of the average Scanty Rainfall: Less than 1 10 mm (Relatively dry day) Moderate Rainfall: 10 25 mm (Relatively wet day) Heavy Rainfall: 25-50 mm (Wet day) Very heavy Rainfall: Greater than 50 mm (Excessively wet day) Probability of 30 %: Isolated (Low Chance) Probability of 40-60 %: Scattered (Moderate Chance) Probability greater than 70%: Widespread (High Chance) 1,4 0,8 0,2-0,4-1 -1,6-2,2-2,8-3,4 Weather Summary-Temperatures: 45 Maximun Temperatures 40 35 30 25 20 15 10 5 0 Monthly Mean Longterm Average Highest Graph 1: Maximum temperatures for December 2016 Average monthly temperatures were within the normal of what is to be expected during the month of December. But there were still cases where the highest maximum temperatures recorded exceeded the normal. The highest temperature recorded was Werda at 41.5 0 C followed by Tsabong 41.3 0 C. 25 20 15 10 5 0 Minimum Temperatures Monthly Mean Longterm Average Lowest Graph 2: Minimum temperatures for October 2016 For the month of December, monthly mean minimum temperatures were higher than the long-term average minimum temperatures. The lowest temperature recorded was 15 0 C at Selibe Phikwe followed by 15.6 0 C at Goodhope. Deviation Max Deviation From Longterm Ave. Temp Deviation Min Graph 3: Deviation From Long-term Average temperatures for December 2016 Generally for December 2016, minimum temperatures were higher than normal. Therefore the mornings were warmer than usual. Maximum temperatures were more or less normal over most areas. Page 2 of 6
RAINFALL SITUATION: During the month of December 2016, much of the rainfall was confined to the northern parts of the Central district and Chobe, where a cumulative rainfall amounts of more than 200mm was recorded over the Northern Central. Nata recorded the highest cumulated rainfall of 218mm, followed by Mmashoro with 171mm. Fig 2: Cumulative rainfall for December 2016 Fig 3: % dep from normal for Dec 2016 Most places recorded below normal rains except for two pockets (southern Kgalagadi and Northern Central) with above normal rains. Satellite reports confirm normal to above normal rainfall (green) over most areas (more than 100%) for December 2016 whilst other pockets of areas had below normal (red) rainfall (Fig 5). Fig 4: Cumulative Satellite Estimated Rainfall (RFE) for November 2016 (Courtesy MESA SADC) Cumulative Satellite Estimated Rainfall (RFE) for December 2016 (Fig 4) shows that the Chobe and Northern parts of the Central District received higher amounts of rainfall whilst the west and southern half received lesser rainfall. This is confirmed also by Fig 2. Note: RFE overestimates the amounts, however the location is precise. For the southern parts of the Kgalagadi, RFE underestimated the rainfall (fig 4 and fig 5 look at southern kgalagadi), due to the fact that it was a warm cloud (Nimbostratus) that was the source of the rainfall recorded here as shown in Fig 2 and fig 3.. Fig 5: % departure from normal Rainfall (RFE) for December 2016 (Courtesy MESA SADC) Page 3 of 6
OCTOBER TO DECMBER 2016: Fig 5a: RFE cumulative rainfall for October to December 2016 (Courtesy MESA SADC) For the Season of October to December 2016, the Satellite estimated rainfall show that the eastern half of Botswana received most of the Rainfall Fig 5a. Whereas the western half received less rainfall. Even the departure from normal map in Fig 5b, shows the eastern half received a cumulative of the three months of above normal rains (green color) and the western half had below normal rains(yellow to red colors). Comparing the satellite estimated rainfall with the surface observations shown in Fig 5c, it is notable that the parts indeed the eastern parts have received more rainfall as compared to the western parts. Fig 5b: % departure from normal Rainfall (RFE) for October to December 2016 (Courtesy MESA SADC) Fig 5c: Cumulative rainfall for October to December VEGETATION CONDITION The vegetation map (Fig 6) shows that it is evident that there is a great improvement in vegetation in most parts of the country from the rainfall that was received during the month of November and December. Vegetation is still showing below normal conditions over pockets of the Chobe, Ngamiland. Ghanzi, and the south-eastern parts of Botswana (red colors). Vegetation takes time to respond to the rainfall, hence an improvement in vegetation is expected after the first dekad of January due to the rainfall that was received during the last dekad of December and first dekad of January. Fig 6: Map showing NDVI difference for the third dekad of November 2016. (Courtesy MESA SADC) Page 4 of 6
DROUGHT MONITORING: The VCI map in Fig 6 shows that even though some areas received good rains, some vegetation hasn t recovered yet from the drought that the country has been experiencing; hence the vegetation is still stressed (red colors). VCI of less than 36% shows extreme drought conditions. But it s evident from the map that some areas have healthy vegetation, and those areas are shown by the green color. Improvement should be noticeable after the first dekad of January, since lots of rains were received during this period (December). Fig 6: Vegetative Condition Index (VCI) map shows healthiness of the vegetation; the value 0(red) shows stressed vegetation, 1(green) is healthy. Anything below 36% is extreme drought (Courtesy MESA SADC) During the month of December, Fig 7 (shows satellite estimated rainfall) shows that areas with a blue color received much above normal rainfall (blue). Which means it was extremely wet in those areas. Most areas had normal rainfall (yellow to green colors), whereas parts the Ngamiland, Gantsi, Kgalagadi and south-eastern parts have had below normal rainfall. Fig 7: Decile Map showing rainfall. (red) is extremely dry and (blue) is extremely wet. (Courtesy MESA SADC) Farmers tips: Farmers to utilize the predicted week (18 th to 22 nd ) of rainfall recess to work on their fields since they have been destructed by the previous wet spells. More rains still predicted during the January to March period. Farmers to adhere to advice given by extension workers. Page 5 of 6
Weather and Climate outlook: TWO WEEK CLIMATE OUTLOOK: (17 th January to 02 nd February 2017) The models show that the country will take a break from the rains that were experienced over the past weeks. From the 18 th till the 22 nd, it will generally just be fair to cloudy conditions with warm temperatures. No thundershowers or rain to be expected during these days. But from the 23 rd, some isolated cases of thundershowers will be experienced over the country. From the 25 th till the 2 nd of February, temperatures will rise again a bit, and thundershowers to be expected over the north and western parts of the country, whereas over the eastern parts it shows a very high likelihood of less rain. Source: GrADS/COLA (NCEP) SUMMARY: Good rains have been received during the October to December 2016 over most parts of the country especially eastern half, as compared with the 2015 season. Water and vegetation situation improved. Rainfall season still progresses and the outlook predicts wet conditions for the last part of the season. Negative impacts on crops anticipated at some places where too much water may cause water logging or flooding in fields. Our Vision: A modern weather service that nurtures and harbours innovation and creativity in the provision of quality weather and climate information Page 6 of 6