Hi Moses, The MOD16 ET algorithm is based on the Penman-Monteith equation (Monteith, 1965) & the algorithm includes the leaf area index (LAI) and the Fraction of Photosynthetically Active Radiation (FPAR). What this means is that MOD16 cannot calculate any ET over water bodies or barren land/rocks/desert areas. You need to have some vegetation for it to work. That is why I made a rectangle around the reservoir that includes surrounding land areas which have some vegetation. If you want to know more about it, please check out my Evapotranspiration video: ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-3r_6il0EViw.html and have a look at the user guide: lpdaac.usgs.gov/documents/494/MOD16_User_Guide_V6.pdf All the best, Andreas
Hi Youssef, I started looking for groundwater in the Middle East about 30 years ago, and then got into GIS and river basin management about 20 years ago. Nowadays I'm doing a lot of capacity building, & am interested in how to monitor water resources using free data and tools. The key for me has always been to go for new opportunities that present themselves, and to never stop learning new skills. Maybe I'll make a video someday, but am too busy at the moment. All the best, Andreas
Please can you help me out on how to calculate evaporation index using water level and evaporation on a dam and its uses. Also, what is the difference between index of evaporation and evaporation? I am working on dam operations and the impact of rainfall extremes.
Hi Triumph, sorry I have not heard of an evaporation index. There is a Precipitation Evaporation Index (PEl) which is calculated from a class A pan evaporation and precipitation data. I don't think it is calculated from dams. Let me know if you find out any further details on this subject. All the best, Andreas PEI: A measure in vertical inches of the average annual abundance of rainfall for a given location. The mean annual class A pan evaporation in inches is subtracted from the normal annual total precipitation in inches to give the precipitation evaporation index. Source: petrowiki.spe.org/Glossary:Precipitation_evaporation_index_(PEl)
in case of off-stream closed loop pumped storage schemes, how the water in the lower reservoir will be filled/replenished if the evaporation is more than precipitation at a given location?
Hi Rathna, most reservoirs in arid regions will have high net evaporation losses - i.e., evaporation from the water surface exceeds the precipitation falling on that surface. If there was no inflow into the reservoirs they would just dry up like a swimming pool which is not topped up at regular intervals. The reason why the reservoirs exist in the first place is that they usually have large catchment areas (aka watersheds). During precipitation events a large proportion of the incoming water from rainfall events becomes surface runoff which finds its way to the reservoir. If the precipitation falls as snow, it will build up in the mountains until it melts in the spring. In both cases, although evapotranspiration may exceed precipitation rates in a catchment, the surface runoff is sufficiently fast so that the a large proportion of the water can reach the reservoir before it is lost to evapotranspiration. If we slow down this process - for example by constructing hundreds of small dams upstream of the reservoir - we may end up with the situation where the reservoir no longer gets filled by surface runoff. All the best, Andreas
Hi Rathna, there are large evaporation losses from all reservoirs. You need to balance the need for these reservoirs and the benefits they provide with the loss of water. When these kind of schemes are designed, evaporation losses must be part of the cost-benefit analysis. If the evaporation losses are too high, there are ways to reduce it - for example floating solar panels - but also this should be part of the design stage of the project. Other options like shade balls are usually not economical over the large reservoir areas. Basically, what is needed is better planning at the design stage, rather than trying to find retroactive solutions. All the best, Andreas
Can I ask a science project question? If I have a huge container let's say circular kids pool, full of sea water and I want to destilate it using the sun, how much freshwater can you expect?
Hi Nelson, Sea water contains about 35g/L of salts (3.5%), which would be left behind if you managed to fully evaporate all the sea water. The volume of fresh water produced should be the same as the initial volume, as the salts dissolved in the sea water don't take up any extra space. In fact adding salts to water can reduce its volume slightly due to a process called electrostriction. So you should end up with a very slightly larger volume of distilled water than the initial volume of sea water. All the best, Andreas See: www.ocean.washington.edu/courses/oc400/Lecture_Notes/CHPT3.pdf
@GeosearchInternational I can't thank you enough for your valuable videos, I have recently become interested in freshwater conservation and desalination because of all the news about draughts in my country. Your videos have been very informative, I hope you don't mind I come back with some more questions.
Hi @getachewbayable. That is a good question, and I spent quite some time looking into it before I developed the methodology of the video. PET is usually measured using open pan evaporation - so evaporation from a water surface, not a land surface. It is really difficult to calculate evaporation from a lake or reservoir as the thickness and temperature of the water column play an important part in controlling ET. Also the air above the water becomes humid, which reduced PET. That is the reason for using 80% of PET. However, there is only one PET which is the maximum possible evapotranspiration rate from any vegetated or open water surface. There are many ways to estimate PET. MODIS uses a modified Penman-Monteith equation (Monteith, 1965) which requires vegetated surfaces. You cannot get any PET/ET values from bare ground. In the video I showed that MODIS gave quite a good correlation with the work done by the USGS at the Kajaki reservoir who used meteorological station data. Hope that answers your question. All the best, Andreas
Dear Andreas, Thank you so much for your response. I appreciate the approach you've suggested. Could you please provide references that have utilized MODIS data, such as MODIS PET, for estimating evaporation on water surfaces? I have tried searching for them, but I couldn't find any relevant literature. I would be grateful for any recommendations you can offer
Hi @getachewbayable, you have my permission to use this RU-vid video as a reference. Also have a look at the description below the video where there is a link to the USGS paper which I use as a comparison to show that it is a reasonable approach to use in areas of the world where meteorological station data is lacking. All the best, Andreas
Dear Andreas, Thank you for your prompt response and for granting permission to reference your RU-vid video. Your approach and explanation style have been enlightening, and I'm eager to delve deeper into this topic. In our pursuit of scientific rigor and credibility, it's imperative to support our methodology with published literature. Your mention of utilizing MOD16 for estimating open-water evaporation has sparked my interest, and I believe referencing a published paper or article would greatly enhance the robustness of our work. Considering your expertise in this field, I kindly request your assistance in locating a published paper or article that demonstrates the application of MOD16 for estimating open water evaporation. Leveraging your knowledge and experience would be invaluable in ensuring that our research is well-supported and respected within the scientific community. Your guidance in this matter would be greatly appreciated. Thank you once again for your generosity in sharing your insights and expertise.
Hi Getachew, the purpose of science is to question everything and find new and innovative approaches to solve problems such as evaporation losses from water bodies. It means that sometimes you end up developing a methodology that nobody else has thought of yet... or maybe they have, but google does not think it is important so we cannot find that special paper. My methodology is inspired by the 2007 USGS report on the Kajaki reservoir, and the only difference is that I used MODIS instead of conventional weather station data. I have a high regard for the USGS, and as my approach gave data comparable to theirs, I think that it is a viable way to estimate evaporation losses. If you do find any other researchers who are using MODIS or other satellite evaporation data for estimating evaporation from water bodies, I would appreciate if you share it with me. Since I made this video I have not come across any myself. I wish you good luck with your research & all the best, Andreas
Good question Moses. I think that MOD16 and the operational simplified surface energy balance (SSEBop) use two different algorithms, so they should give the same trends but obviously not the same outputs. Which one is better will depend on the climatic zone & landuse, but to decide that you will need some real field evaporation data. All the best, Andreas
Hi Madhurjya, for percolation loss estimates you will need to make a full water balance: Inflows-Outflows-Evaporation. You can estimate the evaporation losses using the approach I propose in the video, but you will need accurate measured inflow & outflow data. Could also be supplemented by piezometer data. Good luck & all the best, Andreas
Hi sir I'm interested in calculating evaporation of particular river basin. Can you suggest a software for evaporation calculation. (Software & method)
Hi Manudeep, please see my video on how to get the actual evaporation from a catchment: ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-cUAtYyGI9mI.html All the best, Andreas
Hi Rathna, it would be great to calculate the evaporation rate directly for the reservoir, but the required data is not available and also I have not found any remote sensing product which gives an estimate of transpiration from water bodies. That is the reason why I'm using the potential evapotranspiration (PET) calculated by the MODIS MOD16 algorithm which is a measure of the amount of water that would be lost from the land surface to the atmosphere through evaporation and transpiration, if sufficient water were available. The studies made by the USGS in the wetland Everglades of south Florida in 2006 showed that open water evaporation is about 80% of PET. That is the reason for applying this correction to the PET values. All the best, Andreas
Hi Eng. Zazay, You will find the links to all files in the description below the video. To download in Google Drive, you need to click on the down arrow - on the top right of the screen. All the best, Andreas ******************************************************************* Link to EXCEL data & calculations: bit.ly/3Nbn8rt Link to PPT presentation: bit.ly/3wggQ2O Link to CHIRPS precipitation video: ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-TPSN71KuOUM.html Link to: MODIS Evapotranspiration video: ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-cUAtYyGI9mI.html Link to Reservoir Storage Monitoring video: ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-CRIr5ddmWOY.html *******************************************************************
Hi Youssef, If you have any questions about the video I will try to answer them in this comments section. Sorry I don't have time for emails etc as am very busy at the moment. All the best, Andreas
Hi @@amosgayflorzaizayjr1201 There is no easy way to get this data, as there is always processing involved. I have seen some websites offering radar altimetry data but only for the big reservoirs, and there are some Google Earth Engine tools for processing reservoir areas. To get the storage capacity you need the elevation-area-storage curves which you can only get via digital elevation model or from the reservoir design reports. See: ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-SU3csjbq5lA.html and ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-CRIr5ddmWOY.html All the best, Andreas
