Rainfall Analysis for Yamalur watershed

We had good rainfall (1000mm plus on an average) in 2015. Yet, this summer was harsh for everyone. Would it have helped if we had harvested rainwater? The primary analysis from the project area (Yamalur watershed) observes that there is enough water available for everyone if we do rainwater harvesting leaving excess water to go to the lakes and/or groundwater recharge.  

A watershed is the area of land where all of the water that falls in it and drains off of it goes to a common outlet. Biome Trust has been involved in an action research project called Particpatory Groundwater Mapping in a Yamalur watershed of an area 33.81 sq.km.

Now if we overlap the watershed area over a Google map and the administrative boundary map i.e. BBMP ward map; we would see overlaps with the existing BBMP wards. This helped us in understanding that this watershed consists of 6 BBMP wards- HAL Airport, Bellandur, Marathahalli, HSR layout, Singasandra, Begur, Mangmanapalya and some non-BBMP region still under village Panchayats viz. 4 villages in Halanayakanahalli Gram Panchayat (Halanayakanahalli, Chikkanalli, Chikkanayakanahalli, Hadosiddapura), Rayasandra, Choodasandra, Kodathi.

Rainfall data is very critical for it gives an idea of water falling in an area (inflow), how much is percolating, evaporation percentage, etc. The rainfall related data collection involved contacting Karnataka State Natural Disaster Monitoring Center (KSNDMC). KSNDMC has installed raingauges in 89 BBMP wards so far and almost all village panchayats have these raingauges. We managed to get 2015 rainfall data (daily and monthly) from KSNDMC except Mangmanapalya (where there is no raingauge and so have used closest ward Bommanahalli for the analysis) and villages Rayasandra, Choodasandra, Kodathi. We’ll keep collecting this information. However, some primary analysis throws some interesting results:

Raw data from KSNDMC:

Month	HAL Airport	Bellandur	Marathahalli	HSR Layout	Bommanahalli	Singasandra	Begur	Halanayakanahalli GP
Jan	3	0	3	2.5	3	2	5.5	24
Feb	0	0	0	0	0	0	0	0
Mar	42.5	43.5	44	36.5	45	77.5	16.5	0
Apr	185	118.5	128.5	139.5	129	127.5	102.5	111.5
May	100	64.5	75	102	92	78	100.5	73.5
June	105	132.5	103.5	77	83.5	96.5	100	120.0
July	68.5	55.5	59	58	58	49.5	47.5	55.5
Aug	131.5	187	96.5	169.5	186.5	184.5	138	186.5
Sep	175.5	210.5	164.5	163.5	137.5	175	175	171.5
Oct	144	148	170.5	121	80	117.5	105	160.0
Nov	269.5	200.5	186.5	215.5	229	182.5	171	201.5
Dec	7.5	7.5	3.5	3	3	7	9.62	9.5
Total	1232	1168	1034.5	1088	1046.5	1097.5	971.12	1113.5

Some quick observations:

• On an average it rained 1093.89 mm across the watershed with highest annual rainfall recorded at HAL airport station (1232 mm)
• The lowest was recorded in Begur ward station @971.12 mm
• November being the rainiest month with 200 mm rainfall on average, followed by September with 171 mm and April and October tied at 130 mm

Total area of the watershed	33.81	sq km	33810000	sq m
Total annual rainfall in 2015	1093.89	mm	1.093	m
Total rainfall endowment	36954330	cum	36954330000	L
Per capita requirement	150	LPCD
Total residential population	123780
Total residential water demand	18567000	LPD	6776.955	ML/Yr
Demand as rainfall	200.4423248	mm
Sustainability/Difference between actual rainfall and annual water demand	893.4476752	mm	This much rainfall is available/in excess for the population in the watershed. So if everyone does rainwater harvesting there is enough rainfall for a year for this much residential population
Assumption is that all the rainfall falling is harvested. Rainfall distribution is uniform in the ward.
The per capita requirement in urban area is always found to be more than 150LPCD from our experience.
Commercial establishments are not considered in this calculation

Based on this data, we want to understand some more details:

·         Runoff available- Based on the land use understanding this information would be easy to obtain

·         Recharge- how much of the runoff can be used for recharge, how many wells would be essential

·         Recharge and borewell yields- to determine the efficiency of shallow groundwater recharge and correlation with borewell yields (if any). So we are collecting Static water level (SWL) data from many borewells in the watershed. We use manual and automatic sensors to facilitate the data collection

·         Spatial distribution of rainfall- Bangalore rainfall is varying at the spatial scale so understanding which areas are the high rainfall/low rainfall would help 

·         Microwatershed level rainfall analysis. This yamalur watershed consists of 8 micro-watersheds

We are still learning to analyze this data and would like to hear from you on how best to put this data to use. We would also like to use better visualization. This is what the Participatory Groundwater mapping action research project is attempting to do.