Analysis of the model for a 6.60 m flood The reference year is 1999 with a flood height of 662 cm at the Mopti gauge. The   course   of   the   1999   flood   deviates   quite   significantly   from   the   660   cm   reference   flood.   Indeed,   the   maximum   flood   height of   617   cm   at   Ké   Macina,   which   determines   the   flooding   of   the   upstream   Delta   and   along   the   Diaka,   stands   39   cm   lower   than   the reference   situation   which   should   be   close   to   656   cm. This   deficit   is   compensated   in   Mopti   by   the   stronger   flood   of   the   Bani   River   in Sofara   (+   35   cm   compared   to   the   reference   situation).   However,   the   large   time   lag   between   the   date   of   the   maximum   height   in Sofara   and   that   in   Mopti   (32   days   against   8   days   on   average),   as   well   as   the   difference   in   volume   contributions   between   Niger   and Bani, result in a lower flood than expected in Akka, at the exit of the Debo lake. Landsat images are available for October 18 (198_050), October 27 (197_050 and 051) and November 28: (197_049, 050, 051). The shape file (Synthèse_660_1999) regroups these different situations : The areas in blue on the map are the flooded areas common to the model and the Landsat images. The pink and red areas (Inon_ less) are two different cases of mismatch: 1-   In   pink :   (value   3   of   the   item   carto)   areas   are   shown   vegetation   mosaics   associating   "non   floodable"   vegetation   ( Togge )   or vegetation   potentially   flooded   by   run-off   first,   with   floodable   vegetation   that   were   poorly   flooded   in   1999.      The   former   case includes   the   following   vegetation   mosaics: AG   /   TA, AG   /   TS,   ZB   /   TA,   ZB   /   TS,   P   /   TA,   P   /   TS,   VH   /   TA,   VH   /   TC,   VH   /   TS, ESP   / TA, VSP   / TA,   PAM   / TA,   PAM   / TD,   PAM   / TS),   while   the   latter   case   includes   the   vegetation   mosaics   PAM,   PAN,   PAS, PAM   /   PAN,   PAM   /   PAS,   ZB   /   PAN,   ZB   /   PAS,   PAM   /   P,   PAM   /   VH,   PAM   /   VOR,   P   /   PAN,   P   /   PAS.   Together,   these   mosaics, whose   flood   status   is   regarded   as   "ambiguous",   extend   over   194,160   ha,   of   which   154,331   ha   were   not   flooded   in   1999,   while only 39,829 ha were flooded. 2-   In   red :   (value   2   of   the   carto   item)   are   shown   floodable   vegetation   formations   or   mosaics   that   are   flooded   according   to   the model   but   not   on   Landsat,   amounting   to   143,614   ha.These   non   flooded   areas   distributed   by   flood   depth   level   as   indicated   in the following table: The   majority   of   these   non-flooded   areas   on   the   1999   Landsat   images   are   concentrated   around   Diafarabé,   in   the   Moura plains,   in   the   southern   Pondori   basin   and   in   the   high   plains   of   Sébéra.   They   result,   for   the   first   two   cases,   from   the   water height   deficit   (-39   cm)   noted   at   Ke   Macina   and,   for   the   other   two   cases,   possibly   from   a   poor   flood   provision   by   the   Bani despite    the    higher    than    expected    flood    height    recorded    in    Sofara.    However,    the    last    statement    should    be    carefully considered:   the   Pondori   basin   can   be   flooded   very   early   (in   August)   and   have   only   reduced   flooded   areas   in   the   extreme south by November. In the absence of available images in August, it is difficult to decide.                                         660_1999.gdb.rar  contains : NIV_660 Synthese_660_1999 :   Item inon_543: Value 1: flooded area common to Veg7_21 and Landsat      Value 2: area flooded by the model but not flooded on Landsat Item carto:  Value 1: flooded area common to Veg7_21 and Landsat  Value 2: area flooded by the model, not flooded on Landsat and               composed of floodable vegetation by the river flood.  Value 3: not flooded on Landsat and composed of vegetation  whose flood status is ambiguous (list above)
Gauge  sites   Flood height   in 1999  (cm)   D ates   Reference flood  660 (cm)   Height gap   (cm)   Mopti   662    26 /10   660    + 2    Ké Macina   617    01/10   656    -   39    Sofara   609    24/09   574    +35    Akka   511    23/11   549    - 38 Flood depth (m) Flooded area (ha) [-0.10 à -0.30] 82 090 ]-0.30 à -0.60] 47 258 ]-0.60 à -1.50] 13 625 ]-1.50 à -2.80] 721
On     the     raw     matrix     model (NIV_660),      the      flooded      areas extend   over   1,697,636   ha .   The   area of    floodable    vegetation    units    on VEG7   (without   Farimaké)   measures 1,737,541    ha,     with    the    smoothing done      with      the      500x500      filter causing   an   area   loss   of   2.3%.   The reintroduction      of      these      “lost” flooded    areas    does    not    pose    any difficulties:      it   comes   down   to   using VEG7    directly    by    removing    level 11 (non flooded) (shape VEG7_21).
Overlaying    Landsat    images    in    VEG7_1999    (shape    file VEG7_   1999)   gives   a   fairly   different   representation   of   the   flood, with   a   total   flooded   and/or   heavily   vegetated   area   *   of   1,439,517 ha. *   excluding   areas   heavily   vegetated   by   woody   plants   such   as the Togge in the south of the Delta, for example. The    entire    flooded    /    vegetated    surface    of    VEG7_1999 (Landsat)    is    within    the    limits    of    VEG7_21    so    that    the    area common   to   Landsat   and   VEG7_21   extends   over   1,439,   517   ha . However,   such   a   method   is   only   applicable   in   this   particular   case. For    the    other    flood    levels    analyzed,    the    flood    heights    do    not correspond   to   an   application   limit   of   the   model,   and   we   will   see that   the   flooded   (and   /   or   heavily   vegetated)   surfaces   on   Landsat never   completely   fall   within   the   limits   calculated   by   the   model   for the   maximum   flood   height   at   Mopti.   In   the   model,   there   are   areas flooded   in   addition,   while   some   flooded   areas   are   missed   by   the model.   We   used   the   spatial   operator   “intersect”   between   the   area calculated   by   the   model   and   the   "flooded"   surface   recorded   on Landsat and corrected the errors due to the matrix smoothing. The   area   flooded   on   Landsat   in   1999   was   within   the   area flooded   calculated   with   the   model.   The   ratio   between   the   area flooded    on    Landsat    VEG7_1999    and    on    VEG7_21,    equals 82.8%,   a   relatively   poor   score   explained   in   part   by   the   extent of   vegetation   associations   with   "ambiguous"   flood   status,   and in   part   by   the   characteristics   of   the   1999   flood,   i.e.   the   strong imbalance between the Niger and the Bani floods. The   difference   between   the   model   run   for   a   660   cm   height (VEG7_21)   and   the   year   1999   (VEG7_SF_1999)   as   assessed   by the     spatial     operator     is     recorded     in     the     shape     file     (Inon_ less_660_1999)    and    extends    over    ​​ 298,024    ha ,    calculated    as flooded by the model and not flooded on Landsat images.