sarvesh

The Science of Hydrology in Nepalese Hydropower Projects

Hydrology has long been defined as the science of the movement of water. Hydrology is an important science, especially in case of hydropower projects simply because it determines the scale of the project. Critical analysis of hydrological variables should be conducted prior to any proposed hydropower development.  Such detailed analyses are necessary because a small misinterpretation of data could risk millions of dollars of investment. In other words, if a project is decided with a head of 100 meters and river discharge of 100 cumecs instead of 95 cumecs as a certain dependable flow due to slight difference in methodologies, we could lose a minimum of 1 million US dollars of energy revenue per year in the Nepalese market. What then are the data requirements that we need to take into account when making decisions? The answer lies in critical components of the hydrologic cycle. Some of the most important processes of this are briefly discussed below.

1. Rainfall:

Rainfall affects the overall catchment characteristics from vegetation to channel networks to sediment transportation. Massive loss in reservoir storage is largely due to sediment deposition especially during monsoon in Nepalese rivers. Because rainfall is purely a meteorological phenomenon, it depends mainly on climatic factors like latitude, topography, wind cycle and distance from the sea. How can so many variables then be synchronized into one single mathematical model? The answer lies in long term, research projects in these areas that our country has never commenced.

I would like to highlight two basic kinds of hydrological models. The first one is based on data interpretation and relies on mathematics, probability; reliability analysis and statistical tools to either interpolate or extrapolate the desired result. It is the one largely used in Nepalese context. The other one depends mainly on hydrological processes and are developed for a specific project. Currently we are seeing an increase in the number of Nepalese engineers that are realizing the need for reliable data sources. Our country’s current need is to provide a substantial theoretical basis for designing models that incorporates both of these modeling philosophies so that better decisions can be made with limited risk. Long term data collection of many variables as well as sensitivity analysis for all important variables must be well understood before modeling of such processes can be made.  These long term datasets are also vital for calibrating models as they are developed.

With respect to the rainfall cycle in Nepal, precipitation occurs primarily during the monsoon in which heavy rain falls which frequently result in flash floods around the country. Prior to the monsoon season, soil moisture content and the water table are relatively low. During this period, climatic conditions are relatively dry, yet snow melt fed perennial rivers are recharging the water table.. During the monsoon season, which falls from late June to late September, soil moisture rises and flows in the rivers surge to their highest levels. Snowmelt is also accelerated due to rain on snow conditions over the  mountains. However, not all of the rain water flows as runoff due to considerable volumes of ground water are stored which then raises water table levels. This condition remains throughout the monsoon period.  Subsequent to the monsoon season, the river and water table levels fall again and the processes repeat themselves.

How do we model rainfall then? Could we do it in a macro scale or do we study separately for different catchments? Nepalese meteorologists must play a role in answering these important questions.

2. Snow hydrology

This field is largely understudied and has not attracted the attention of Nepalese hydrologists. As such, very little data exists.  Large volumes of water are stored in the snow of our extensive mountainous regions. This snowpack serves as a balancing reservoir in our river catchments. Snow feeds both surface runoff and ground water which in turn feeds our rivers. Snowmelt is the primary input into rivers during our dry seasons prior to the monsoon. During the winter monsoon, snowmelt adds flow to rivers. As a result Nepal has experienced unexpected lowering in duration of load shedding during February and March every year. No past data analysis can be found in connection to the snow hydrology in context of Nepal. Department of Meteorology and Hydrology has established quite a few snow measuring stations but, unfortunately the data are either hard to find or not reliable.

3. Ground water hydrology

Ground water hydrology is also in an early stage of development in Nepal. There has been little to no study in this field. Experts seem to be little concerned with impact of ground water fluctuations during project development and implementation. One aspect that is often forgotten is that ground water has a high significance in project reliability both technically are financially. When we talk about reliability of flow in rivers, in many ways we are talking about seasonal water table variations. For a better understanding of river hydrology, ground water hydrology and geohydrology needs to be well understood. Ground water contour maps need to be plotted over time to better understand groundwater flow paths and potential runoff volumes. Geologists and hydrologists need to work together to develop robust datasets that can help guide hydropower development decisions.

If we observe the trend of project development in Nepal, we find out that there is less importance given to the very inputs of design, i.e. hydrology. Rain, snowmelt and groundwater all serve as inputs, and the magnitude, frequency and duration of these inputs ultimately determines a project’s future. If we arrive at conclusions insufficient information, substantial sums of money can be lost.  Due to the uncertainty in many hydrologic inputs, decision making in hydropower development is very tricky business. Past trends show that different consultants arrive at different conclusions for the same river inputs. Why is there a discrepancy? When we talk about flood analysis, the results often vary significantly. We always tend to forget that it is the magnitude of the same flood that determines around 50% of the total project costs.

4. Evapotranspiration

Especially in case of reservoir projects and projects with either large or very small catchments, there is a critical need to study the net evapotranspiration (ET) of a project’s catchment. People don’t realize that increases in evapotranspiration, results in tangible losses in energy production. ET on the other hand, also affects the rainfall distribution. This is one of the reasons why we need to study this field for a hydropower project. Projects with large catchments lose large amounts energy through ET while projects with relatively smaller catchments may lose small energy but it affects the overall rates of return.

5. Snow sublimation

If we observe snow peaked mountains everyday around 1 o’clock in the afternoon, we would easily know how snow sublimation changes the water balance of a catchment. Currently, we have no data that indicate how much water evaporates annually by this process.

6. Snow melt

While snow melt is especially high during monsoons, this process provides a steady input of water to catchments by snowmelt all around the year.

7. Glacier formation

Glaciers, their movements and parameters affecting them are important to be studied. GLOF is one of the major challenges and having to incorporate it into a project is a matter of great risk and challenge both for designers and developers.

8. Topography and geology

Aspects of rocks, rock types, slopes and faces of mountains all affect the hydrology of a catchment. River catchments of Nepal are difficult to model. There are catchments with fairly regular rainfall patterns towards the south east near Bay of Bengal and there are those which fall on a rain shadow region on north and west side. For example, the Tamakoshi river catchment receives more rainfall and snowfall than Kaligandaki catchment. If we observe snow peaked mountains over Koshi River in Google earth, they are whiter with large glaciers reaching up to China. If the same is observed for Kaligandaki River, the catchment acts like a large desert. One of the other ways of how topography affects the hydrology is that mountains with gentle slopes on the northern sides retain more snow and vegetation than those facing south.

The challenge is to be able to choose an optimum design among almost infinite number of choices. Ultimately government policies, personal and project developer’s interests determine the fate of a project more than what is known about a potential project’s runoff characteristics. An understanding of a project’s  risk helps in decision making and arriving at conclusions. This understanding also aids the government and private developers in their decisions as well. The risk calculation should however be backed up with substantial practicability both in terms of theory and data.