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International Institute for Geo-Information Science and Earth Observation (ITC) 2016

Development of a method to validate agronomic drought as assessed by NDVI time series through satellite derived estimates of rainfall

Garcia Velez, L. (Laura)

Titre : Development of a method to validate agronomic drought as assessed by NDVI time series through satellite derived estimates of rainfall.

Auteur : Garcia Velez, L. (Laura)

Etablissement de soutenance : University of Twente International Institute for Geo-Information Science and Earth Observation (ITC)

Grade : Master of Science in Geo-Information Science and Earth Observation 2016

Résumé
Considering that the agronomic sector in Ethiopia is highly threatened by drought events, different risk management strategies as weather insurance products have been promoted during the last years. In this context, the use of satellite information is promising as it provides measurements which are spatially continuous and with high temporal resolution . The GIACIS model ( Geodata for Innovative Agricultural Credit Insurance Schemes ) uses the Normalize Difference Vegetation Index (NDVI) as a proxy to monitor the occurrence of agronomic drought events. Considering potential limitations of this model to accurately identify the affected areas, the present study explore options of validation through the use of satellite derived estimates of rainfall (meteorological drought). In detail, the validation is carried out over specific Crop Production System Zones (CPS) and its associated growing calendars, using the concept of effective rainfall ( available water after losses due to evaporation, deep percolation and runoff ). The datasets used are 10- day SPOT- VGT (NDVI) and 10- day CHIRPS (rainfall) imagery. At first, the study involves ; i) derivation of the optimal long term NDVI- rainfall relationships at selected CPS using moving average and Pearson correlation, and ii) improvements in the annual NDVI- rainfall relationship to account for effective rainfall at the region of Tigray (CPS- 7) using fixed thresholds . As a result, i) the long term relationships and associated optimal lags portray the general variability that exist among the selected CPS due to the different macro-scale factors that determine the availability of water (e.g. climate, land cover, management practices and soils ) , and ii) t he annual NDVI- effective rainfall relationship of the CPS- 7 displays high variability at the end of the growing season, which might be related with other environmental factors that might have higher influence than rainfall during this period , or the need of more refinement in the estimation of effective rainfall. In this study, the exploration of agreement between agronomic and meteorological drought for the CPS- 7 considers the above findings and involves ; i) calculation of NDVI and rainfall anomalies through the threshold level method for the years 1999- 2014 (CPS level), and ii) Kappa analysis of the spatial agreement between NDVI and rainfall anomalies (pixel level , using a categorical classification ). As a result, i) the NDVI and rainfall anomalies agree for 13 out of 16 years in a general linear relationship, ii) the spatial agreement between the anomaly categories is only 21.5% better than chance, which points out that still there are a lot of limitations in the accurate representation of effective rainfall through the use of satellite estimates. To finalize, the study explores the agreement between agronomic drought (NDVI anomalies) and modelled soil moisture estimates at a selected pixel . A very simple model based on the Thornthwaite’s water balance technique is implemented in a daily basis for the period 1999-2014, using daily CHIRPS ( rainfall) and 10- day MARS- ECMWF (potential evapotranspiration) data. Although the results are not conclusive, the model has been show promising to include processes that also might trigger the development of an agronomic drought ( e.g. distribution of rainfall events and its impact through specific periods of the growing season).

Mots clés : agronomic drought, Crop Production System Zone, effective rainfall, meteorological drought, NDVI, soil moisture

Version intégrale (ITC)

Page publiée le 3 février 2018