Spatio-temporal dynamics of reservoir (de-)commissioning in Ceará, NE Brazil
- 1University of Potsdam, Environmental Science and Geography, Hydrology and climatology, Germany (arlena.brosinsky@uni-potsdam.de)
- 2Helmholtz Centre Potsdam - GFZ, Department of Geodesy, Section Remote Sensing and Geoinformatics
- 3Federal Institute of Ceará - IFCE, Laboratório Ambiente e Sociedade, Ceará, Brazil
- 4Federal University of Ceará - UFC, Department of Agricultural Engineering, Ceará, Brazil
The North East of Brazil is characterized by a high spatial, seasonal and inter-annual variability of rainfall. The climate is semiarid with pronounced wet and dry seasons. In the federal state of Ceará, water supply for the dry season is ensured by the construction of reservoirs of various sizes. However, most of these reservoirs were built without documentation by the water management authorities. Thus, there is no complete state-wide inventory of reservoirs in Ceará. Using the satellite based global surface water dataset (GSW), this study aimed at investigating (i) the location, (ii) size, (iii) commissioning and decommissioning year as well as (iv) spatio-temporal dynamics of reservoirs in Ceará for the period 1984 - 2018.
Based on the maximum water extent of the GSW dataset, 17 919 reservoirs larger than 90 x 90 m were detected in Ceará (ii), which could be confirmed with an accuracy of 87 % for a validation dataset including 157 reservoirs regularly monitored by the State Agency for Meteorology and Water Resources in Ceará (FUNCEME). Thereby, reservoirs with a maximum water extent < 2.05 ha (category 1) form the largest and reservoirs with a maximum water extent > 50 ha (category 6) the smallest share in number of all detected reservoirs (i). In turn, reservoirs of category 6 contribute most to the water storage capacity while category 1 reservoirs contribute least. For the estimation of (de-)commissioning years (iii), the GSW monthly water history dataset was used. The years of commissioning and decommissioning were determined as the first and last year, respectively, of water being detected. The calculated commissioning years were validated against the FUNCEME dataset, whereas to date there is no validation data to confirm the decommissioning of reservoirs. The analysis of spatio-temporal dynamics of reservoir commissioning and decommissioning (iv) indicate a spatially variable increase of reservoirs until approximately 2010, followed by an intensive decrease until 2015 (analysis of extended study period currently ongoing).
Deviations in terms of commissioning years are mainly small and can be attributed to uncertainties inherent to satellite observations. Up to 2010, the detected spatio-temporal variability can be largely attributed to droughts which occurred in most parts of the state in several years and particularly severe since 2012. However, the maximum rate of large strategic reservoir construction by the public sector (for which records exist) decreased since the 1990’s, which is attributed to a shift in the adaptation strategy to water scarcity, from structural measures (construction of dams) to governance measures (water management and control). From the data presented here, it seems that this trend likewise occurred on the level of small reservoirs for which no records existed so far.
How to cite: Brosinsky, A., Timmke, S., Foerster, S., Francke, T., Medeiros, P., and de Araújo, J. C.: Spatio-temporal dynamics of reservoir (de-)commissioning in Ceará, NE Brazil, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8068, https://doi.org/10.5194/egusphere-egu2020-8068, 2020
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Hi Arlena,
Thanks for the presentation. Interesting work, and great to see use of the GSWE.
My question in the chat: @Arlena, what are some possible drivers for reservoir decommissioning?
Your response: @Daryl - If you look at the presented material, you will notice a major increase in decommissioning towards the end of the study period; this is partly due to a major drought occurring from 2012 and partly due to study design (especially in the last year of observation, many reservoirs may have fallen dry but will be (partly) refilled in the next wet season). However, there is also a possibility of breaking earth dams in wet years (as happened in 2020). But there may be additional explanations ... any suggestions?
Me again: @Arlena. Thanks. In North America and Europe, reservoir decommissioning now outweighs construction. Reasons include safety, ageing (redundant) insfrastructure, ecological reasons (usually driven by EU's Water Framework Directive). I wonder if any of these apply to Brazil... Happy to discuss further.
To continue:
I was quite surprised to see so much apparent 'decommissioning' in Ceará, as this bucks the global trend for global dam construction outside of North America and Europe. If you would like any literature on this, please let me know. In your case, I would think the drivers would be different or absent entirely. From your answer, it sounds as if many of the reservoir disappearances might just be drying. Or perhaps they are filling with sediment (as Pedro et al. suggest EGU2020-10199). If they are filling with sediment, that does mean that can be excavated/dredged and rehabilitated? Or do the owners simply abandon them (which could be quite a safety risk and environmental harm)? Or are the structures breached to allow flow to pass, with other structures left intact? Clearly this is hard to validate without field trips or high resolution imagery.
From my perspective, it would be fascinating to discover whether there is genuine decommissioning (dam removal) in Brazil.
Hi Daryl,
thank you for your interest in our study and your great comments and questions, also during the chat session.
Initially, our aim was to use the GSWE data to assess the commissioning dates of reservoirs in Ceará (past 1984). There are records for the large ones but many thousands of smaller dams in remote areas were built without documentation and thus there is no complete state-wide inventory. We were quite surprised to observe so many reservoirs "disappearing". This is work in progress and we have not been able to validate our observations (by means of high resolution data and in the field; it's on our agenda for the coming month);
With regard to the suspected reasons for "decommissioning", there are several:
1. our observations are based entirely on optical satellite data, which can differentate "no data", "water" and "no water". So yes, "no water" for several seasons can mean simply that the reservoir dried up and did not refill in the following wet season(s) (as José Carlos de Araújo pointed out during the chat session, the reservoir network is quite complex and some reservoirs might refill in regular wet seasons while others only refill in very wet years (after periods of drought)). This will be the case especially towards the end of our study period, with a major drought from 2012-2017. I expect that many of the reservoirs will "reappear" and am excited to analyse the GSWE update until 2018 (currently work in progress).
2. However, the "decommissioning" started to increase well before the 2012 drought when there were still regular amounts of rain. I assume there are several reasons. Our partners in Brazil indicated the breaking of earth dams in wet years to be a potential explanation for some "real" decommissioning; I was wondering whether an (observed) decrease in water quality and potential increase in macrophyte growth could also results in observations of "decomissioning" by our definition (I have seen entire reservoirs overgrown which would likely not be classified as "water" by the GSWE); we have not heard of active reservoir removal.
3. Siltation is an issue in the area but from the figures I know it is much more a problem of decreases in water quality than in quantity; I do not know of any reservoirs that have actually filled up with sediment as is the case in other regions (@Pedro or José Carlos, please correct me if I am wrong here).
I agree that it's a very fascinating subject and would be happy to keep in touch.
All the best from sunny Potsdam, Arlena
Thanks Arlena and Pedro for your responses. Where can I find out about your future work and publications? I've followed Arlena on ResearchGate.
Dear Daryl,
please find bellow a list of selected recent publications about the reservoir network. The work presented by Arlena is still unpublished, but we may update you as it progresses.
My email address is phamedeiros@ifce.edu.br, in case you want a more direct contact. I believe that we can benefit from further discussion.
Medeiros, P.H.A.; Sivapalan, M. (accepted) From hard to soft path solutions: slow-fast dynamics of human adaptation to droughts. Hydrological Sciences Journal.
Lira, C.C.S., Medeiros, P.H.A., Lima Neto, I.E. (2020). Modelling the impact of sediment management on the trophic state of a tropical reservoir with high water storage variations. Anais da Academia Brasileira de Ciências, 92(1): e20181169. https://doi.org/10.1590/0001-3765202020181169
Rodrigues, I.S.; Ramalho, G.L.B.; Medeiros, P.H.A. (2020) Potential of floating photovoltaic plant in a tropical reservoir in Brazil. Journal of Environmental Planning and Management. https://doi.org/10.1080/09640568.2020.1719824
Brasil, P.; Medeiros, P.H.A. (2020) NeStRes – model for operation of Non-Strategic Reservoirs for irrigation in drylands: model description and application to a semiarid basin. Water Resources Management, 34,195-210. https://doi.org/10.1007/s11269-019-02438-x
Braga, B.B.; Carvalho, T.R.A.; Brosinsky, A.; Förster, S.; Medeiros, P.H.A. (2019). From waste to resource: cost-benefit analysis of reservoir sediment reuse for soil fertilization in a semiarid catchment. Science of the Total Environment, 670, 158-169. https://doi.org/10.1016/j.scitotenv.2019.03.083
Nascimento, A.T.P.; Cavalcanti, N.H.M.; Castro, B.P.L.; Medeiros, P.H.A. (2019) Decentralized water supply by reservoir network reduces power demand for water distribution in a semi-arid basin. Hydrological Sciences Journal, 64(1), 80-91. https://doi.org/10.1080/02626667.2019.1566728
Pereira, B.S.; Medeiros, P.H.A.; Francke, T.; Ramalho, G.B.; Foerster, S.; de Araújo, J.C. (2019) Assessment of the geometry and volumes of small surface water reservoirs in a semiarid region with high reservoir density by remote sensing. Hydrological Sciences Journal, 64(1), 66-79. https://doi.org/10.1080/02626667.2019.1566727
Zhang, S.; Foerster, S.; Medeiros, P.; de Araújo, J.C.; Waske, B. (2018) Effective water surface mapping in macrophyte-covered reservoirs in NE Brazil based on TerraSAR-X time series. International Journal of Applied Earth Observation and Geoinformation, 69, 41-55. https://doi.org/10.1016/j.jag.2018.02.014
Mamede, G.L.; Güntner, A.; Medeiros, P.H.A.; de Araújo, J.C.; Bronstert, A. (2018) Modeling the effect of multiple reservoirs on water and sediment dynamics in a semiarid catchment in Brazil. Journal of Hydrologic Engineering, 23, 05018020. https://doi.org/10.1061/(ASCE)HE.1943-5584.0001701
Zhang, S.; Foerster, S.; Medeiros, P.H.A.; de Araújo, J.C.; Motagh, M.; Waske, B. (2016) Bathymetric survey of water reservoirs in north-eastern Brazil based on TanDEM-X satellite data. Science of The Total Environment, 571, 575-593. https://doi.org/10.1016/j.scitotenv.2016.07.024
Dear Daryl, thank you very much for your comment and the interest in our work.
Arlena explained very well the main factors that might have led to reservoirs “disappearance” (not necessarily decommissioning) in the study area, and I would like to add further information.
The 2012-2017 drought was the most severe in last century, and even the very large reservoirs (> 1 x 10^9 m³ storage capacity) dried out. In general, reservoirs have recovered water storage in the last years, and we expect that the extension of the study period mentioned by Arlena shows this hydrological effect.
Full macrophyte coverage of some reservoirs should play an important role on the water mismapping. Water quality monitoring by the Ceará State Water Resources Management Company has shown an intense process of eutrophication, particularly during the abovementioned drought. Recently, some works were conducted on the macrophyte mapping (see, for instance, Zhang et al. (2018) Effective water surface mapping in macrophyte-covered reservoirs in NE Brazil based on TerraSAR-X time series. International Journal of Applied Earth Observation and Geoinformation, doi: 10.1016/j.jag.2018.02.014), indicating that it is a major issue.
As highlighted by Arlena, siltation is not expected to completely fill reservoirs with sediments in a few decades. Previous studies conducted by prof. de Araújo (co-author of this abstract) indicate that siltation reduces the storage capacity of reservoirs by an average rate of 1.6% per decade in the study region.
A feature that we do observe during years with above-average rainfall is the break of earth dams, particularly very small reservoirs (farm dams). This might explain some of the reservoir decommissioning. Also, some water use sectors (large cities and industries, for instance) speech against the small reservoirs, claiming that they retain the water, impacting the inflow to the large strategic ones. However, as the small reservoirs serve as important water sources to rural population and contribute to their livelihood, decommissioning has not been adopted as a public policy. What we do observe is a reduction on the rate of reservoir commissioning since the 1990’s.
At last, we hope to be able to do field verifications soon, in order to validate the GSWE-based estimations and explain the factors leading to reservoirs’ disappearance.