Streamflow monitoring in the Hanumante Watershed
Flooding is regarded as one of the critical concerns in urban areas of Nepal, especially Kathmandu Valley (KV). The frequency of urban flooding in the KV has increased dramatically over the recent years owing to abrupt population growth, and unplanned urbanization (World Bank, 2017). Apart from that, river pollution is widely prevalent in the KV, which has ultimately affected the urban ecology of the streams. Among the different tributaries of the Bagmati river in the KV, the Hanumante river is highly prone to urban flooding, and water pollution. Although the river has several cultural/irrigational significance (Shrestha 2007; Sada, 2012) and its upstream is a critical source of water to the inhabitants, the river has become tremendously stressed in recent years. The river has experienced the two largest incidents of flooding in the last decade (2015 and 2018), which had several detrimental impacts.
Figure 1: Flooding Hanumante Watershed in 2015 (left) and 2018 (right).
Furthermore, the flood risk seems to be emerging with each passing year. This can be attributed to the encroachment of the floodplains for settlement purposes and the government lags for developing/implementing proper land use policy (Prajapati, 2018). The direct discharge of untreated household sewage and industrial effluents, pesticides from agricultural runoff has led the Hanumante river to be severely polluted. These two critical issues of the Hanumante river have posed significant socio-economic challenges/impacts in the livelihood of the people living nearby; majorly by the inundation of the agricultural and settlement area, health hazards associated with pollution of the river, and so on. As such, proper hydrometeorological data are crucial to understand the flood dynamics, propose effective mitigation measures to the associated stakeholders/policymakers, and manage the river in a sustainable manner. There are already four rainfall stations installed by the Department of Hydrology and Meteorology (DHM) in different places of the Hanumante Watershed (Changunarayan, Nangkhel, Nagarkot and Suryabinayak). However, much effort has not been made in the last decade to collect the water level/streamflow/quality data of the Hanumante river.
Realizing the need for hydrological monitoring in the Hanumante Watershed, Smartphones For Water (S4W-Nepal) has been gathering hydro-meteorological data of the Hanumante river since 2019 with the help of Citizen Scientists (CS) and young researchers. S4W-Nepal has been monitoring the stream water level using staff gauges, and bimonthly stream discharge using Acoustic Doppler Velocimeter at the 12 different sites of the Hanumante river (Figure 1). Along with that, S4W-Nepal has been collecting a few physicochemical water quality parameters such as temperature, pH, Electrical Conductivity (EC), Total Dissolved Solids (TDS), and dissolved oxygen using respective portable water quality meters. Besides, S4W-Nepal in collaboration with the High Powered Committee for Integrated Development of Bagmati Civilization (HPCIDBC) have been monitoring the important water quality parameters, including pH, Turbidity, Total Suspended Solids (TSS), Biological Oxygen Demand (BOD), Chemical Oxygen Demand (COD), Fecal coliform, and total phosphate) from nine different sites of Hanumante river.
Hanumante Watershed receives 80% of total annual rainfall within the monsoon season i.e., June-September. The seasonal variation of streamflow in the Hanumante river from 2019 to 2021 is clearly presented in Figure 2. It is evident that the streamflow in the Hanumante river is majorly dependent upon monsoonal rainfall. The discharge of all three major tributaries is less than 0.2 m3/s in all seasons, which increases to 1.2 m3/s or more after the confluence in monsoon and post-monsoon seasons. However, during pre-monsoon and winter, the discharge of the stream remains moreover constant i.e., less than 0.4 m3/s from upstream to downstream, which is likely due to the insignificant rainfall, extraction of water for irrigation and construction purposes, and groundwater percolation (Wang, 2013). Besides, a recent study has suggested that the estimated return period of the water level of about 3.5m in the Hanumante river is ten years (Kindermaan, 2020).
As per the recent data by HPCIDBC at nine different sites, the overall water quality of Hanumante river in post-monsoon of 2021 was found to be adversely degraded. It was found that the water quality parameters for the majority of sites exceeded the National Water Quality Guidelines for Irrigation Water/Aquaculture. This suggests that the water quality of the Hanumante is not suitable for the irrigational purposes and aquaculture. Further, the sites located in the dense settlement areas of the Bhaktapur district (downstream of Hanumante river) were found to be highly polluted, which might be due to the multiple anthropogenic influences.
In the coming days, S4W-Nepal intends to continue monitoring the streamflow, water level, and quality even more intensely and generate some meaningful inferences/information, which will support the management of Hanumante river. The regular monitoring of Hanumante river is crucial to determine flood frequency, return level and maximum flood level, which can assist in designing flood mitigation plans, and reducing the dreadful impacts of flooding Moreover, local government/organizations, academic institutions, and responsible authorities should work together with specific goals and plans for better management of the river. The management of the river in a sustainable manner along with local participation should be the utmost priority of all the concerned stakeholders.
Kindermann, P.E., Brouwer, W.S., van Hamel, A., van Haren, M., Verboeket, R.P., Nane, G.F., Lakhe, H., Prajapati, R. and Davids, J.C.\ (2020) Return level analysis of the hanumante river using structured expert judgment: A reconstruction of historical water levels. Water, 12(11), pp.3229. Available from DOI: https://doi.org/10.3390/w12113229
Prajapati, R., Raj Thapa, B. and Talchabhadel, R (2018) What flooded Bhaktapur? My Republica, 17 July 2018.
Sada, R. (2012) Hanumante River: Emerging uses, competition and implications. Journal of Science and Engineering, 1, pp.17-24.
Wang, S.Y., Yoon, J.H., Gillies, R.R. and Cho, C. (2013) What caused the winter drought in western Nepal during recent years?. Journal of Climate, 26(21), pp.8241-8256.
World Bank (2017) Land Use Planning for Urban Flood Risk Management. Urban Floods Community of Practice Knowledge Notes. The World Bank, 2017.