Taking a look at global news, one is stormed with alarming news of climate change all over. Rainfall periods are increasingly becoming shorter but more intense all over. The worst-hit areas are arid and semi-arid. The timing for these changes is quite brutal. This is because the world’s population is at an all-time high whereas the demand for water consumption is significantly rising. Therefore, man has adopted some ingenious ways to store water or tap from extra sources.
It’s crucial to estimate evaporation levels correctly for the proper dam maintenance. As research intensifies and new methods or old ones are being revived, several threats present themselves. For surface storage, evaporation and seepage are proving to be a menace. Evaporation especially will differ at different times of the year and in different regions.
While there exist a lot of evaporation management articles, there is less content dedicated to drier areas where evaporation happens to be worse. They experience longer sun-periods it’s worth coming up with the estimation models for the correct measurements. Due to the differing extent of water loss, several methods have been adopted to estimate evaporation based on this metric.
Methods of Evaporation Estimation Models in Semi-arid Regions
As already said, there are different methods used across the world to estimate evaporation from a given area and at a certain period. These will have different accuracies and will also depend on the surrounding region.
Some models used to estimate evaporation include:
- Makkink Method
- Hamon Method
- Brutsaert-Stickler Method
- Penman-Monteith Method
- Jensen-Haise Method
- De-Bruin-Kejiman Method
- Granger-Grey Method
- Morton’s CRAE Method
The factors considered when using the above methods to estimate evaporation include radiation, temperature, energy budget, and mass transfer. Nonetheless, in this article, we will focus on four commonly-used methods and our area of study will be India. Next to China, India is seen to have the biggest world population. As per-capita income increases, there is an incremental need for cleaner and safer water sources. This is part of the reason that pushed us to study India.
These methods are:
- Bowen-ratio energy balance (BREB)
- Mass Balance (MB)
- Priestley-Taylor (PT)
- Pan-evaporation method (PE).
Studies carried out over the years have proven that the energy budget methods yield more accuracy than the rest. This is because they are data-intensive and thus have a lot of considerations. That notwithstanding, they are still considered to be costly and are mostly used over small areas.
Scientists and researchers usually have a hard time determining which method to use over which meteorological region. In arid and semi-arid areas, however, rainfall excess tends to establish irrigation water management schemes. Understanding the importance of these storage facilities to these people, researchers are forced to establish the most viable means of estimating evaporation. The success in establishing this will eventually lead to an increased reliance on the diverse components of the water balance equation.
The Purpose of this Study
Now that we know all this, what is the target of conducting this study? We aim to compare and contrast the most used methods to estimate evaporation in semi-arid regions and establish the most reliable one. Once this is done, we also seek to establish while evolving the appropriate techniques for parameter estimation of the established method using limited data then analyze their reliability. Our focus is primarily on small water reservoirs used for irrigation in India.
- The Browen-Ratio Energy Balance Method (BREB)
Equation Used:
Where:
- EBR Is the evaporation rate
- Rn is the net radiation on water surface i.e. incoming radiation – reflected radiation
- G Is the heat lost or gained by the upper layer of the water body
- Qb is the heat flux into the bottom of the water body
- Qa is the energy advection into the water reservoir
- λ is the latent heat of water evaporating
- is the Bowen ratio which is dimensionless
Some researchers have condensed this formula by eliminating the heat flux and the energy advection going into the water body. This has not only proven to be expensive for them but has also reduced the expected accuracy by 7%. Nonetheless, using the original form of the equation was found to be not only portable but also quite simple.
The pitfall in using this method to estimate evaporation is that it yielded less accurate data under stable conditions. Stable atmospheric conditions would yield relative errors of 21% whereas, in unstable conditions, the errors would surmount to 11%. During the daytime, accuracy was found to be much higher under unstable conditions. It is generally thought that the source of the errors is from the insufficient fetch which brings about local advective conditions.
To establish the water surface emissivity, researchers needed to derive the incoming solar or short wave radiation. Details such as the fraction obscured by clouds need to be included to reach fairer results. Moreover, the duration of sunlight and atmospheric pressure are also meaningful variables to be included. This helps in determining the actual vapor pressure.
- The Mass Transfer (MT) Method
This approach of measuring evaporation rates makes various assumptions. Evaporation rates are assumed to be a linear function of wind speed. Since there is free surface water evaporation in this experiment, the Dalton equation is used. This is summarized as:
A being the total surface area of the water body. Alternatively, the mass transfer coefficient can also be determined from the slope of the straight line that passes through the origin.
- The Priestly-Taylor (PT) Method
The PT method has a wide instance of application. It has been tried and tested in many investigations and is considered to be the most promising of the four. It is a simplification of the combination of aerodynamics and radiation. The PT coefficient is considered to be the incline of saturated vapor temperature at the water temperature versus the temperature of the curve. It is usually considered to be 1.26.
- The Pan Evaporation Technique (PE)
Most scholars consider this approach to be the most standard and most commonly used. Even with this, certain assumptions have to be made and it also has its degree of error.
Assessing the Study Area and Parameters Used
Almost 39% of India can be categorized as semiarid. This covers an area of approximately 329 million hectares while 15.1% is considered arid. This study was conducted on the Bundi district located in the south-eastern parts of Rajasthan, India. The area has an elevation of 252 m above sea level and average rainfalls o 750mm. Summer temperatures stand at a mean of 35°C while winters remain cooler at about 15°C.
The area has small reservoirs with sizes ranging from 2000 – 10000m2. These are widely used during the non-monsoon months for irrigation purposes. People have become increasingly dependent on these irrigation schemes as a source of livelihood. As a result, Participatory Irrigation Management schemes have been undertaken by state and central governments to preserve the efficiency of accumulated water storage. The most common methods are increasing the vegetation cover and treatment of watersheds to reduce erosion.
A pond was then picked and daily climatic conditions recorded as the methods to estimate evaporation were used. To come up with the best model, it was imperative to specify the input data for each method before the research. Furthermore, the model parameters had to be established and the model performance verified to reach conclusive results.
Discussing the Results
Beginning with the air-water temperature model, temperatures were determined 10mm below the water surface. This happened for a period of 155days. This was then compared with air temperature and the statistics fitted into three models:
- Liner Model
- Logarithmic Model
- Power Model
The liner model ranked first in terms of correlation and was closely followed by the power model. The linear model is thus found to be the best to use in this analysis.
In comparing the daily energy fluxes, both short and longwave radiation appear to follow similar patterns. The heat energy factor is found to account for the highest heat energy flux followed by sensible heat factor. When there is a smaller value for the stored heat flux factor, it shows there was a minimum impact of the energy stored on evaporation.
When total results are combined and tabulated, they were compared against each other for the four models. The coefficients for the four methods were as follows;
- of BREB was estimated at 0.07
- of MT ranged at 2.35
- of PT was found to be 1.31
- KP of PE at 0.65
The performance of PE, PT, and BRED was found to be comparable. Evaporation rates in MT were found to vary significantly from year to year. The results were arrived at by comparing the statistical properties. The PT needed limited data to reach its findings as opposed to the BREB and was generally found to be the most effective and accurate.
The MT model is discouraged due to its relative variations. That said, the relative comparability of the other three methods was easily notable. As such, both the Bowen ratio and the PT coefficient were found effective in the semiarid regions of India. What’s more, it was established that a linear regression model was the best to use. This is to draw the correlation between air and water temperature. This data is important in predicting the missing water temperature from the air temperature.
