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Validation of the water productivity under soil moisture stress for different agro-ecological zones of Rwanda

Niyonshuti, Belle Ange (2021) Validation of the water productivity under soil moisture stress for different agro-ecological zones of Rwanda.

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Abstract:Soil moisture availability is one of the most important parameters governing biomass production and actual evapotranspiration (ETa), key parameters used to determine the Water Productivity provided by FAO-WaPOR. The triangle method is one of approaches incorporating soil moisture availability as one of the factors which could reduce the ETa due to environmental stress. This approach is based on correlating Normalized Difference Vegetation Index (NDVI) and Land Surface Temperature (LST) retrieved from remote sensing to the ground measured soil moisture. For this study, no in-situ soil moisture was available, hence observed soil moisture from SMOS level 2 was used. The triangle method was investigated on four landcover types ( Akagera savanna, Nyungwe forest, tea and eucalyptus) located in three distinctive major agro-ecological zones of Rwanda. The triangle method was best applied for the sample areas especially for Akagera savanna as the three most important properties of the triangle method were well observed (cold edge, peak of the triangle and warm edge). On the other hand, the methodology was not applicable to Nyungwe forest because there is no edge to be found (no warm or cold edge in the scatter plot). There is no limiting factor related to soil moisture as this is a mountain rain forest, hence it was not considered for further analysis. The results of the triangle method indicated that all sample areas are under soil moisture stress conditions for the period investigated especially Akagera savanna with soil moisture stress factor of 0.15 . Validation of FAO-WaPOR ETIa was done by investigating the application of the triangle method for different landcover types in order to evaluate this aspect of FAO-WAPOR ETIa methodology and by comparing FAO-WaPOR ETIa to the ETa derived from two different methods. The first ETa was derived using Penman-Monteith method and it is based on climatological data, the second ETa was derived using DATTUTDUT model which is solely based on a LST image. The comparative analysis indicated that the three methods are comparable with slightly deviating values less than 1mm/day for all the sample areas probably due to the difference in spatial resolution. Both Penman-Monteith method and DATTUTDUT model ETa were in good agreement with the ETIa provided by FAO-WaPOR. Soil moisture stress affected FAO-WaPOR-ETIa by overestimating ETIa values especially when having lower soil moisture stress values. The highest overestimation in ETIa was found in Akagera savanna area which has the lowest soil moisture stress value. Therefore, the sensitivity of ETIa to stress is very high when having lower soil moisture stress values. FAO-WaPOR does not provide some of the actual data (LST, NDVI and soil moisture stress factor) used to produce their ETIa product, therefore more detailed assessment on the evaluation of the triangle method input parameters in the aspect of FAO-WaPOR ETIa methodology was not possible. To accurately assess FAO-WaPOR ETIa product, further study on these parameters is recommended . The output of this study is essential to FAO-WaPOR, as it could provide them useful information on how good or uncertain their products are.
Item Type:Essay (Master)
Faculty:ITC: Faculty of Geo-information Science and Earth Observation
Programme:Geoinformation Science and Earth Observation MSc (75014)
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