Rain Water Does Not Drain from Hills to the Streams, Even after Heavy Rainfall [50 cms]

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Rain Water Does Not Drain from Hills to the Streams, Even after Heavy Rainfall [50 cms]

We have a situation in the part of western ghats, Coimbatore, where a specific region of hill ranges, water drain is NILL from the hill to the respective stream paths.

We can approximate the average rainfall on the specific hill catchment area is about 50 - 100 cms in a period of 2 months.

Have visually ensured, no deforestation or no developments of civil constructions.

We, the public, realized the gradual decrease in water flow to the stream from past 10 years, but the heavy monsoon shower helps to revive the water beds by getting the life of those streams back to normal. But past couple of years despite normal rainfall, no water drained from the mountain range.

The stretch of mountain range impacted by this change is about 10 miles, which is about coverage of 2000 hectares of agricultural land.

I seek technical assistance and help to understand the cause of the absence of water drainage and revive the situation.

CO_ORDINATES :

 10°55'13.43"N

 76°47'32.54"E

 

Thanks

Murthi

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8 Answers

  1. Hello, I have seen this in Guatemala, near our Atlantic coast. To make sure of what you are dealing with, you need to look farther and closer. Measure and have actual flow measurements. Verify if flow speed is increased due to River bedrock changes. Also check other streams nearby. On our case, we were able to find that the stream of interest had constant flow even with similar conditions you describe, but found another spring, coming out of nearby caves on limestone, that one had the changes of flow. We had been paying attention to the wrong springs. Measure and make sure you have data and compare to historical records. Look for tendencies in year to year flow. There's no right answer without actual rain and flow measurements. Best wishes.

  2. Dear Thanikachalamurthy Aruchamy

    The main reasons for decline of water flow in the mountainous river is decline of base flow condition and increase of runoff. The specific hill region mostly covered by charnockites and associated weathered rocks. Overall, in most of the South Indian river system surface runoff increased as instantaneous flow, Irrespective of maintenance of forest cover and not disturbing by any civil constructions. The main reason what I could understand the river bed might have silted with eroded material, which increase surface flow and not favour for infiltration and subsequent base flow. Artificially we can revive the rivers by making infiltration pit along the rivers. This practice provide good results as adopted in river rejuvenation project by art of living , Bangalore. 

    regards

    S.Anbazhagan 

     

     

    2 Comments

    1. I'd like to point out that its removal of the native forest cover that changes streamflow from lesser instantaneous flow and longer baseflow, to greater flashiness and shorter baseflow duration. In addition, high pumping rates in the floodplains can lower the overall water table, that can dry up springs and small order streams, in turn decreasing flow in the rivers as they emerge from the hills. Plantation forestry ( exotic spp like eucalyptus, australian pine, wattle, albizia etc) can have high evapotranspiration and lesser rain interception by the open canopy  and subsequent decrease in infiltration into the groundwater. Soil erosion is also higher in plantation forestry than native shola forests, even though lower than hillslope farming.

    2. Thank you Anbazhagan for the response.

      I am not sure about the material property of the hill, but would like to know the cause of your mentioned case - "silted with eroded material".

      Also, you have meant the infiltration pit along the rivers - But my case is specifically on the hill ranges - Do you suggest for such pits on the path of the stream.

      It would be great, if I have the contacts of such experts to take it forward.

       

      Thanks

      Murthi

  3. As most Hydrologists will tell you that runoff is a function  of the Intensity, Duration and Frequency (IDF) characteristics of rainfall, but stream flow is also affected by things such as Base Flow, evaporation and seepage rates.  Seepage rates are determined by substrate material and evaporation by tree cover and cloud cover etc.  However, at the moment I'd like to concentrate on Base Flow.  Again, base flow is influenced by the geological material and seepage rates, but it is also a function of the water content in these materials too.  Increasing tree cover will actually decrease contributions to Base Flow  and runoff enormously as trees can be looked at as large water pumps delivering transpiration to the skies while actively growing. This effect tends to flatten off as the forested areas mature.  Without sufficient recharge to the aquifers on the mountains (and particularly if some of this is used by well use etc.) base flow will decline in times of drought and may not recover for many years.  In dry times the only thing that keeps streams flowing is Base Flow (ground water originating from aqufiers) and if you look at Base Flow as a financial ledger with both credits and debits to its balance sheet, declining credits and and increases debits in the form of increasing dry periods without recharge is a receipy for declining stream flow...perhaps for years into the future.  I am Lippy.  Hear me roar.

    1 Comment

    1. Thanks Kevin Linton  for your insights.

      Just want to quote an incident, when there was a heavy rain [did not have the measurements] couple of years back, the few of the mentioned streams were flooding out only for the rained days.

      While I accept this period may be a heavy recharge of water from the rainfall .But, I am not expecting the flow during the dry days - But the downstream flow of heavy rain.

      Aqufers were existing, but hardly functions now.

      Please let me know, what is the right approach to address this.

       

      Thanks

      Murthi

  4. All the answers below are possibilities.  If the bedrock contains a limestone horizon which can be prone to the formation of sinkholes, one or more sinkhole collapses over the past decade could be diverting surface water from the monsoon into a fractured aquifer or cavern system.  Examination of the ground surface in the vicinity of the major streams for evidence of ancient sinkholes may be possible but this effort will unfortunately be impeded by the forest cover you describe.  This situation, if true, would be very difficult to fix.  "Civilization exists by geologic consent, subject to change without notice".  [Will Durant] Good luck. 

    J. Gusek

    1 Comment

  5. Dear Mr. Aruchamy

     

    I am not familiar with the subsoil and the geological substratum of your region and it would be probably best to consult a local geologist / hydro-geologist.

    However, your description of the phenomenon leads me to think that the percolation pattern in the area has changed and water now percolates to deeper geological strata, below the river network in your (upper ?) part of the catchment. Changes in the percolation patterns might be caused by agricultural practices as deep ploughing which can disturb impermeable soil layers or – if not caused by human intervention – by seismic activity. The fact that there is runoff during periods of intense rainfall might be explained with a limited percolation capacity of the subsoil / the geological substratum.

     

    With kind regards

     

    Richard Kuntner 

     

    1 Comment

    1. Hi Richard Kuntner ,

      Thanks for the response.

      The agricultural activities is 1 mile or more from the catchment area in the hill - Still there could be an impact due to deep ploughing ?

      1 Comment reply

      1. Dear Mr. Aruchamy

        Deep ploughing might eventually have an influence if "intermediate flow" from the hills to a stream network passes below the agricultural area. I am however feeling a little bit like speculating as I am not familiar with the catchment.

        May I suggest that you delineate on the map you added the catchment, the stream network and the agricultural area and eventually mark a neighbouring catchment where no changes in the hydrologic response has been observed ? This could eventually help all participants in the discussion to better orientate themselves.

        By the way: I had a closer look at your map and found the exact location you indicated using Google Earth - this map might give some hints on how the basin used to work - if the stream networks indicated on the map are accurate and I read the map correctly - a stream network originating in the hills disappears in the plane and another starts in the plane close to the bottom of the hills. (Both networks are ot the west of the location you indicated by the coordinates). In the first case there might be a percolation to deeper geological strata and in the second case the network might be alimented by intermediate flow from the hills. Nevertheless, here again I am still guessing.

         

  6. You mentioned that the forest is undisturbed.  Could it be that the forest is healthier or more massive than in the past?  Perhaps it is soaking up the rainfall for the benefit of the plants.

    1 Comment

  7. Are, or were there, any springs in the area? If so, what has been their condition over the same period? I'm curious as to whether historical streamflows in the stream are from direct runoff, or from  groundwater exfiltration. If it's the latter, it's possible that something (increased groundwater pumping for agriculture, etc) has lowered  the groundwater table down below the elevation of the stream, thereby not allowing the groundwater to charge the stream. 

    1 Comment

    1. Hi Daniel Frazier, P.E. ,

      Thanks for the response.

      I do had the same doubt, but unfortunately we don't have the data for the springs.

      The streamflows are mostly from the direct runoff, very few are of exfiltration from groundwater - I did not see such broad area of aquifers to store water for run off.

      Indeed, there was massive exploitation of ground water for agriculture in this area.

      As I said earlier to above query - The catchment area is 2 miles far and 100 meters different in altitude to the agricultural area for borewell - Will that still impact the runoff ?

      Thanks

      Murthi

  8. Perhaps you can explain your quandary more clearly - you do mention that heavy monsoons help to revive streamflow, which indicates that some rainfall is being drained into the stream.

    In some cases, bedrock layers in a valley may direct the percolated rain in the valley  to flow to a neighboring catchment. Otherwise, rainfall in a catchment would certainly reach the stream. Unless there is a lot of pumping (which you mention is not happening).

    2 Comments

    1. Hi Amartya Saha ,

      Thanks for the response.

      Please note that the heavy monsoon have revived streamflow in the past drought [10 years before], but it did not happen in the few years [last 5 years] and we see decline in the flow of the stream from hills and reached to NIL this year.

      We do suspect the deviation in the stream flow direction as a cause - We are yet to verify due to heavy rain at those locations and we are not sure who is the right authority to get this details.

      I did mentioned no destruction in the mountain ranges - But as mentioned in the earlier queries - lot of pumping in the plateau and the plains.

      Please note that we have one of the groundwater depletion rate in our district.

      Thanks

      Murthi

      1 Comment reply

      1. Monitoring groundwater in wells, together with rainfall measurements and streamflow can answer whether heavy pumping in the plains may be lowering down the water table below streambed level. However, that usually requires instrumentation which may not be available. Replacing the native vegetation by fast growing exotic trees ( eucalyptus specius, albizzia etc) can lead to high rates o water uptake, and indeed have been seen to lower groundwater and dry up springs in the Nilgiris. I assume you are referring to the Palnis, near Palani and Pollachi  ? 

    2. Could  you please provide more details about your location and provide us a map of the area. Then I might be able to help you in a meaningful way.

      1 Comment reply