Person: Chahor, Youssef
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Chahor
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Youssef
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Proyectos e Ingeniería Rural
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0000-0003-1046-5349
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Publication Open Access Analysis of the hydrological and erosive behavior of agrararian watersheds in Navarre (Spain) by experimentation and modelling(2017) Chahor, Youssef; Casalí Sarasíbar, Javier; Giménez Díaz, Rafael; Proyectos e Ingeniería Rural; Landa Ingeniaritza eta ProiektuakIt is general knowledge that agrarian activities causes important effects on the environment. It is therefore necessary to better understand these effects as well as propose controlling measures. To this end, the information obtained from agrarian watersheds (experimental) is very valuable. However, simulation models enable the extension of this knowledge to wider zones, once calibrated in limited zones such as the mentioned experimental watersheds. Computer tools allow for medium- and long- term analysis of the implications of changes in use and management of soil, which is essential to advance in the control of environmental problems derived from agrarian activities. Among the issues caused or enhanced by agrarian activities, hydric erosion of the soil must be mentioned, which is the main focus of this thesis, in its traditionally recognized categories: sheet, rill, and gully erosion. The main objectives of this thesis are: evaluate the AnnAGNPS model as a tool to estimate runoff and sediment exports in a small experimental watershed (Latxaga, 207 ha), with homogeneous soil use and representative of wider zones of Navarre; carry out the same exercise in a larger watershed (Cemborain, 50 km2), much more complex regarding relief, climate, and soil use and management; explore the capacity of the model to simulate scenarios, aimed at its utilization as a management tool; evaluate, in an extensive zone with high gully activity, the capacity of the APEGT module, integrated within AnnAGNPS, to identify the locations of the hydrographic watersheds where the apparition of ephemeral gullies (EGs) are expected; evaluate the observation capacity of orthophotos to characterize EG erosion in wide zones and throughout long periods of time, providing data on the main characteristics of the phenomenon in a zone of Navarre, and evaluate the TIEGEM model, integrated within AnnAGNPS, to predict EG erosion in Navarre. The evaluation of AnnAGNPS for runoff and sediment exports was carried out firstly in the experimental Latxaga watershed, almost entirely cultivated with winter cereal, and secondly in the Cemborain watershed (analysis limited to runoff), with 24% of its surface cultivated (the remaining 40% and 30% are respectively, forests and brushland). Results indicate that good calibration was performed for runoff for both watersheds, with satisfactory behavior of the model when validating the first case, but unsatisfactory for the second case. Calibration considered variation of CN only, searching for adequate simulation at monthly levels, then escalated to seasonal and annual scales. Calibration consisted of adjusting the initial CN values, as well as adding new CN values during Winter and Spring. Nevertheless, it was necessary to utilize unrealistic CN values in some cases, with could raise questions on the validity of the model. On one hand, this could be due to the necessity of adjusting the elevated infiltration observed in the zones during dry periods only by CN; infiltration was much higher than expected in accordance with soil texture. This could have happened due to the presence of preferential flows, verified in the zone. On the other hand, adjustment of simulated runoff in Winter and Spring presented the opposite effect (although less pronounced): it was necessary to increase CN values as a consequence of: i) very low CN values at the beginning of Autumn, and ii) decrease of infiltration by preferential flow paths due to the expansion of soil (consequence of elevated soil humidity during Winter). This behavior seems to evidence the own limitations of the CN method for small watersheds or in those where local or peculiar phenomena could have great transcendence and not be adequately simulated. Evaluation of the model for the simulation of sediment exports resulted less satisfactory. Calibration, after an innovative sensibility analysis where six parameters that intervened in the process were identified, provided good results, aimed at the optimization of annual simulations. However, validation provided poor results. The complex erosion processes of the watershed, despite its reduced size, and the necessity of utilizing more detailed topographic information, could be the cause of such poor results in the validation stage. The use of more complex submodels or models is suggested, within the AnnAGNPS environment, such as CONCEPTS, which enables a more detailed simulation of sediment dynamics. Nevertheless, these verifications seem to decrease the applicability of AnnAGNPS for our conditions, given the high requirements for data and detailed calculations, inadequate for general use in practice. In any case, this tool can be very useful when long term trends are necessary, rather than absolute values. In this sense, it must be mentioned that scenario analysis provides very realistic results. The APEGT module for the location of EG heads, based on the CTI (Compound Topographic Index), more specifically on its critical value, CTIc, has been evaluated for a homogeneous zone regarding climate, and soil use and management. Great variability has obtained throughout the years analyzed. This indicates that the processes that control the formation of EGs are much more complex than what is generally admitted, including subsurface flows (exfiltration of seepage), preferential flow paths, soil humidity regime conditions (influence in turn by its position within the environment), etc. The variability observed resulted similar across the years, which enables the affirmation that CTIc can be representative and therefore useful to estimate the average location of EGs in a homogeneous zone at the long term. This coincides and is coherent with the objectives established by the designers of AnnAGNPS. For the correct analysis of the CTI method, it was necessary to identify two values: CTIc1 corresponds to the watershed zones where special conditions are present that favor new heads, while CTIc2 enables identification of the zones where head migration is not possible anymore. Multitemporal analysis of orthophotos, at detailed scale and throughout long periods of time, obtained at the same moments each year, permitted a good characterization of the EG erosion phenomenon in an extensive area with significant savings in resources. In a similar reflection of what was previously exposed, the low repetitiveness in the apparition of EGs (it is rare that gullies form in the same microwathershed) also highlights the complexity of the phenomenon, controlled by very different factors. The evaluation of the AnnAGNPS model was carried out in four small watersheds affected by EGs, and the study presented herein, although modest, is the most complete carried out up to date. This highlights the scarcity of existing data to proceed with these evaluations, evidencing the difficulties in data obtainment and providing value to methodological approaches such as those aforementioned (orthophotos). Calibration was carried out only by variation of the τc (critical shear stress) parameter, aimed at an adequate simulation of the moment of EG apparition as well as their shape and volume. It must be mentioned that, during the calibration phase, adequate identification of the events that caused the apparition of each EGs was accomplished by the model. However, estimation of volume has not been very successful. This has been fundamentally conditioned by the poor predictions of gully length.