The hydraulic head refers to a **measurement of the energy**, in water within rivers, streams, or lakes. It represents the water level in a flowing body of water. Simply put it measures the height of a column of water above a point and is commonly expressed in meters (or feet in the US). When the water level or hydraulic head is higher, there is energy available at that particular location.

In facilities, the amount of harnessed energy depends on the disparity between the upstream headwater level in the reservoir and the downstream tailwater level below the dam. The distinction referred to here is called the hydraulic head difference. It indicates the **amount of energy that can be converted into electricity using turbines and generators. **Further calculations reveal that besides distance various losses called head losses also influence energy harnessing.

Because the water in the reservoir being higher up has greater horizontal gravitational potential energy than that in the tail race. The gravitational potential energy of the reservoir water that comes down through the penstocks leads to the creation of energy required to move the turbines and produce electricity. By employing Bernoulli’s equation, the procedure is depicted herein. Generally, a** hydraulic head is equivalent to one gravity energy unit – in this case, it will be water storage.**

The hydroelectric power equation utilizes the value for the hydraulic head to give the estimated available power. This factor plays a role in the equation represented as follows:

**P = ρQgΔh**

Where:

- P – The rate at which power is being calculated and measures are expressed in joules per second known as W.
- Δh – Represents the hydraulic head difference across the dam or turbine measured in meters.
- ρ – It depends on the fluid’s density measured in kg/m3.
- Q – Denotes the volumetric discharge or flow rate of water measured in meters per second (m3/s).
- g – Represents gravity’s acceleration, measured in meters per second squared.

This equation states that a** larger difference in head leads to a potential for mechanical energy stored in reservoirs.**

**Types of Hydraulic Head**

There are three classifications of dams based on their hydraulic head differences; medium and low. Further details, about these hydraulic head types will be discussed subsequently.

**1. High Head**

When there is a **difference in head of more than one hundred meters** it is known as high-head. Unlike the previous one, the water flowing in this plant is usually drawn from heights above its surroundings, meaning it requires less volume to generate similar energy. As water flows through such systems, it does not flow at a high pace and hence small turbines are required.

The water also has to travel for some distance using a long penstock, since even a shorter turbine and a narrower penstock will lead to the formation of air pockets that reduce efficiency. Most often, big hydropower plants belong to the high and medium heads.

**2. Medium Head**

In mediumhead systems, **head difference ranges from 10 – 100 meters. **There is less elevation drop compared with a high-head dam when considering the penstock in a medium-head dam. This kind of dam takes advantage of a considerable quantity of water and a great loss of height of the said water.

**3. Low Head**

Systems normally have** less than 10 meters** of low-head dams. Due to that, low-head hydro turbines are usually applied in operations like run-of-the-river systems which entails moving rivers without significant rise.

A typical low-head system transports a lot of water, therefore bigger turbines are required to transform water energy into power efficiently. No damming of water is required for these installations as less water needs to be stored.

**Also Read:** 5 Major Advantages and Disadvantages of Hydroelectric Energy

**Hydraulic Head Losses**

These losses **occur due to friction within pipes**. Accounting for these head losses reduces the amount of energy in water. This adjusted value, for a head considering losses, is referred to as an effective head.

The effective head is equal to the gross head less all losses of head. All hydroelectric facilities produce head loss which is divided into major and minor head loss. The hydraulic head losses are then measured, calculated, and expressed on an equal footing with the hydraulic head itself, e.g. in **terms of the equivalent height of the water meter**. This requires one to deduct, from total gross power, the power wasted due to head loss; then one gets actual net power one can draw. The equation can be shown as:

**P _{net} = P_{gross}− P_{loss}**

**Also Read: **What is Conventional Hydroelectric Plant?

**Types of Head Losses**

The types of head losses are:

**Major head losses:** The vast majority of major head losses come out of friction in the pipes, and they run over long distances of the pipeline for instance in the penstock.

**Minor head losses:** Any other source of minor head losses apart from frictional ones. In essence, any place a pipe bends or the movement of the water is changed has some kind of loss which is called minor loss.

However, there exist cases whereby the major head losses are less than the minor ones but they sum up to determine the total power output of a hydraulic dam, unlike what their names suggest.

**Must Read:** What is Hydraulic Fracturing?