What you want to do is take a look at TerrainTypes.xml. In the top terrain you'll find
 <TerrainType InternalName="Land">
    <DisplayName>Plains</DisplayName>
    <MaxHeight>50</MaxHeight>
    <MinHeight>50</MinHeight>
    <MovementCost>8</MovementCost>
    <TerrainMovementNote>1x Movement</TerrainMovementNote>
    <Category>Land</Category>
    <IsTerrainFeature>0</IsTerrainFeature>
    <IsBaseTerrain>1</IsBaseTerrain>
    <PassableByLandUnits>1</PassableByLandUnits>
    <PassableByWaterUnits>0</PassableByWaterUnits>
    <PassableByAirUnits>1</PassableByAirUnits>
    <SuitableForStartingLocation>1</SuitableForStartingLocation>
    <Indestructable>0</Indestructable>
    <MinTilesPerSplat>0</MinTilesPerSplat>
    <MaxTilesPerSplat>0</MaxTilesPerSplat>
    <MinBrushRadius>3</MinBrushRadius>
    <MaxBrushRadius>6</MaxBrushRadius>
    <PlacementOrder>0</PlacementOrder>
    <SupportsSubTerrain>false</SupportsSubTerrain>
    <IsBorderTerrain>0</IsBorderTerrain>
    <CannotBorder>Category:Water</CannotBorder>
    <MapGenPlaceChance>100</MapGenPlaceChance>
    <GameModifier InternalName="CityTileYield_Fertile">
      <ModType>Resource</ModType>
      <Attribute>TileYieldGrain</Attribute>
      <Value>1.5</Value>
      <PerTurn>1</PerTurn>
    </GameModifier>
    <TerrainTypeEnvData>
      <Environment>Barren</Environment>
      <Texture>gfx/terrain/barren_ground_rough.dds</Texture>
      <TextureWeight>1.0000</TextureWeight>
      <TerrainPropData>P_Berren_Land_01,100</TerrainPropData>
      <TerrainPropData>P_Berren_Land_02,100</TerrainPropData>
      <TerrainPropData>P_Berren_Land_03,100</TerrainPropData>
      <TerrainPropData>P_Berren_Land_04,100</TerrainPropData>
      <TerrainPropData>P_Berren_Land_05,100</TerrainPropData>
      <TerrainPropData>P_Berren_Land_06,100</TerrainPropData>
      <TerrainPropData>P_Berren_Land_07,100</TerrainPropData>
      <TerrainPropData>P_Berren_Land_08,100</TerrainPropData>
      <TacticalMap>T_Barren_Land_01</TacticalMap>
      <TacticalMap>T_Barren_Land_02</TacticalMap>
      <DistanceFogColor>94,89,76</DistanceFogColor>
      <ClothMapColor>205,185,130</ClothMapColor>
    </TerrainTypeEnvData>
    <TerrainTypeEnvData>
      <Environment>Dead</Environment>
      <GrantsTileYields>1</GrantsTileYields>
      <Texture>gfx/terrain/fallen_ground.dds</Texture>
      <TerrainPropData>P_Fallen_Land_01,100</TerrainPropData>
      <TerrainPropData>P_Fallen_Land_02,100</TerrainPropData>
      <TerrainPropData>P_Fallen_Land_03,100</TerrainPropData>
      <TerrainPropData>P_Fallen_Land_04,100</TerrainPropData>
      <TerrainPropData>P_Fallen_Land_05,100</TerrainPropData>
      <TerrainPropData>P_Fallen_Land_06,100</TerrainPropData>
      <TerrainPropData>P_Fallen_Land_07,100</TerrainPropData>
      <TextureWeight>1.0000</TextureWeight>
      <TacticalMap>T_Fallen_Land_01</TacticalMap>
      <TacticalMap>T_Fallen_Land_02</TacticalMap>
      <TacticalMap>T_Fallen_Land_03</TacticalMap>
      <DistanceFogColor>83,65,88</DistanceFogColor>
      <ClothMapColor>165,180,190</ClothMapColor>
    </TerrainTypeEnvData>
    <TerrainTypeEnvData>
      <Environment>Grassland</Environment>
      <GrantsTileYields>1</GrantsTileYields>
      <Texture>gfx/terrain/light_grass.dds</Texture>
      <TextureWeight>1.0000</TextureWeight>
      <TerrainPropData>P_Grassland_01,100</TerrainPropData>
      <TerrainPropData>P_Grassland_02,50</TerrainPropData>
      <TerrainPropData>P_Grassland_03,30</TerrainPropData>
      <TerrainPropData>P_Grassland_04,100</TerrainPropData>
      <TerrainPropData>P_Grassland_05,90</TerrainPropData>
      <TerrainPropData>P_Grassland_06,20</TerrainPropData>
      <TacticalMap>T_Grassland_Land_01</TacticalMap>
      <TacticalMap>T_Grassland_Land_02</TacticalMap>
      <DistanceFogColor>200,220,228</DistanceFogColor>
      <ClothMapColor>180,185,120</ClothMapColor>
    </TerrainTypeEnvData>
  </TerrainType>
I bolded some stuff there, those correspond to the various environments a "Land" terrain tile can have (barren = brown, dead = empire, grassland = kingdom). In particular interest to you is the <Texture>gfx/terrain/fallen_ground.dds</Texture> tag, which you'll want to change for something that looks like a desert.
Replace that for all the different terraintypes inside the <TerrainTypeEnvData> where <Environment>Dead</Environment>.