scholarly journals Model-based study of the role of rainfall and land use–land cover in the changes in the occurrence and intensity of Niger red floods in Niamey between 1953 and 2012

2016 ◽  
Vol 20 (7) ◽  
pp. 2841-2859 ◽  
Author(s):  
Claire Casse ◽  
Marielle Gosset ◽  
Théo Vischel ◽  
Guillaume Quantin ◽  
Bachir Alkali Tanimoun

Abstract. Since 1950, the Niger River basin has gone through three main climatic periods: a wet period (1950–1960), an extended drought (1970–1980) and since 1990 a recent partial recovery of annual rainfall. Hydrological changes co-occur with these rainfall fluctuations. In most of the basin, the rainfall deficit caused an enhanced discharge deficit, but in the Sahelian region the runoff increased despite the rainfall deficit. Since 2000 the Sahelian part of the Niger has been hit by an increase of flood hazards during the so-called red flood period. In Niamey city, the highest river levels and the longest flooded period ever recorded occurred in 2003, 2010, 2012 and 2013, with heavy casualties and property damage. The reasons for these changes, and the relative role of climate versus land use–land cover (LULC) changes are still debated and are investigated in this paper. The evolution of the Niger red flood in Niamey from 1950 to 2012 is analysed based on long-term records of rainfall (three data sets based on in situ and/or satellite data) and discharge, and a hydrological model. The model is first run with the present LULC conditions in order to analyse solely the effect of rainfall variability. The impact of LULC and drainage area modification is investigated in a second step. The simulations based on the current surface conditions are able to reproduce the observed trend in the red flood occurrence and intensity since the 1980s. This has been verified with three independent rainfall data sets and implies that rainfall variability is the main driver for the red flood intensification observed over the last 30 years. The simulation results since 1953 have revealed that LULC and drainage area changes need to be invoked to explain the changes over a 60-year period.

2015 ◽  
Vol 12 (11) ◽  
pp. 12039-12087 ◽  
Author(s):  
C. Casse ◽  
M. Gosset ◽  
T. Vischel ◽  
G. Quantin ◽  
B. A. Tanimoun

Abstract. Since 1950, the Niger River basin went through 3 main climatic periods: a wet period (1950–1960), an extended drought (1970–1980) and since 1990 a partial recovery of the rainfall. Hydrological changes co-occur with these rainfall fluctuations. In most of the basin the rainfall deficit caused an enhanced discharge deficit, but in the Sahelian region the runoff increased despite the rainfall deficit. Since 2000, the Sahelian part of the Niger has been hit by an increase of flood hazards during the so-called Red flood period. In Niamey city, the highest river levels and the longest flooded period ever recorded occurred in 2003, 2010, 2012 and 2013, with heavy casualties and property damage. The reasons for these changes, and the relative role of climate vs. Land Use Land Cover (LULC) changes are still debated and are investigated in this paper. The evolution of the Niger Red flood in Niamey from 1950 to 2012 is analysed based on long term records of rainfall (three data sets based on in situ and/or satellite data) and discharge, and a hydrological model. The model is first run with present LULC conditions in order to analyse solely the effect of rainfall variability. The impact of LULC and drainage area modification is investigated in a second step. The simulations based on the current surface conditions are able to reproduce the observed trend in Red flood occurrence and intensity since the 1980s. This has been verified with three independent rainfall data sets and implies that rainfall variability is the main driver for the Red flood intensification observed over the last 30 years. The simulation results since 1953 reveals that LULC and drainage area changes need to be invoked to explain the changes over a 60 year period.


2017 ◽  
Vol 33 (11) ◽  
pp. 1202-1222 ◽  
Author(s):  
Sudhir Kumar Singh ◽  
Prosper Basommi Laari ◽  
Sk. Mustak ◽  
Prashant K. Srivastava ◽  
Szilárd Szabó

Author(s):  
C. C. Fonte ◽  
L. See ◽  
J. C. Laso-Bayas ◽  
M. Lesiv ◽  
S. Fritz

Abstract. Traditionally the accuracy assessment of a hard raster-based land use land cover (LULC) map uses a reference data set that contains one LULC class per pixel, which is the class that has the largest area in each pixel. However, when mixed pixels exist in the reference data, this is a simplification of reality that has implications for both the accuracy assessment and subsequent applications of LULC maps, such as area estimation. This paper demonstrates how the use of class proportions in the reference data set can be used easily within regular accuracy assessment procedures and how the use of class proportions can affect the final accuracy assessment. Using the CORINE land cover map (CLC) and the more detailed Urban Atlas (UA), two accuracy assessments of the raster version of CLC were undertaken using UA as the reference and considering for each pixel: (i) the class proportions retained from the UA; and (ii) the class with the majority area. The results show that for the study area and the classes considered here, all accuracy indices decrease when the class proportions are considered in the reference database, achieving a maximum difference of 16% between the two approaches. This demonstrates that if the UA is considered as representing reality, then the true accuracy of CLC is lower than the value obtained when using the reference data set that assigns only one class to each pixel. Arguments for and against using class proportions in reference data sets are then provided and discussed.


2021 ◽  
Author(s):  
Roland Yonaba ◽  
Angelbert Chabi Biaou ◽  
Mahamadou Koita ◽  
Tazen Fowé ◽  
Adjadi Lawani Mounirou ◽  
...  

<p>Land use/land cover (LULC) change is a major factor affecting the hydrological response at the watershed scale. However, hydrological modelling, in its current practice, is usually carried using a single and static LULC layer for simulation runs over long periods. Eventually, this approach leads to failure in accounting for LULC spatial and temporal changes as well as non-linear impacts on simulated outputs. Besides, in the typical case of Sahelian hydrosystems, previous modelling attempts based on this approach failed at reproducing the well-known Sahelian hydrological paradox which occurred in the area during the period 1970-1990. This study aims at assessing the added value of dynamical integration of LULC changes in hydrological modelling of surface runoff in Sahelian hydrosystems. The Tougou watershed (37 km²), located in Northern Burkina Faso is selected as a case study. LULC maps of the watershed are produced from 1952 to 2017 from the processing of Landsat satellite images. The SWAT (Soil & Water Assessment Tool) model, using the SCS-CN method (for surface runoff estimation), is calibrated and validated using observed runoff data collected over the period 2004-2018. The calibration/validation is carried using LULC maps of the watershed in 1999, 2009 and 2017, dynamically integrated into the model using a specific land use update module. Further, the calibrated model parameters helped in the reconstitution of surface runoff over the historical period 1952-2005 and integrating dynamically LULC maps in 1952, 1973, 1986 and 1999. The results showed that between the periods 1952-1968 (P1) and 1986-2005 (P3), the average annual rainfall decreased by 87.9 mm while paradoxically, average annual runoff increased by 1 mm. Further analysis revealed that the increase in runoff is mainly attributed to LULC changes (+647%) which offsets the effect of the decrease in rainfall (-547%). From the analysis of LULC maps, it was found that from P1 to P3 periods, the decrease in natural vegetation (CN = 67.3 ± 5.7) by 40%, replaced by bare and degraded soils (CN = 83.8 ± 2.5) explained the observed increase in surface runoff potential of the watershed, as shown by their calibrated CN values. These findings are reminiscent of the Sahelian hydrological paradox reported in the literature and provide evidence of the sensitivity of surface runoff to LULC changes. Overall, the results call to hydrologists, water resources planners and managers, regarding the advantages of coupling LULC changes in hydrological modelling. Also, the study advocates for the development of integrated modelling platforms integrating both LULC changes and hydrological modelling to allow a better understanding and the more accurate long-term forecasting of water resources, in particular in the case of Sahelian hydrosystems.</p><p><strong>Keywords:</strong> Dynamic LULC input, Hydrological modelling, Surface runoff, SWAT model, Burkina Faso, Sahelian paradox.</p>


2007 ◽  
Vol 164 (8-9) ◽  
pp. 1789-1809 ◽  
Author(s):  
Joseph G. Alfieri ◽  
Dev Niyogi ◽  
Margaret A. LeMone ◽  
Fei Chen ◽  
Souleymane Fall

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