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>.