duck

float[, ,] splatmapData = new float[terrainData.alphamapWidth, terrainData.alphamapHeight, terrainData.alphamapLayers];

for (int y = 0; y < terrainData.alphamapHeight; y++)
{
    for (int x = 0; x < terrainData.alphamapWidth; x++)
    {

        // calculate denseness, steepness and wetness for this position, and then...

        Vector4 splat = new Vector4(1, 0, 0, 0);
        splat = Vector4.Lerp(splat, new Vector4(0, 1, 0, 0), denseness);
        splat = Vector4.Lerp(splat, new Vector4(0, 0, 1, 0), steepness);
        splat = Vector4.Lerp(splat, new Vector4(0, 0, 0, 1), wetness);
        splat.Normalize();

        splatmapData[x, y, 0] = splat.x;
        splatmapData[x, y, 1] = splat.y;
        splatmapData[x, y, 2] = splat.z;
        splatmapData[x, y, 3] = splat.w;
    }
}

terrainData.SetAlphamaps(0, 0, splatmapData);