If the Son canal system has more water, why are Bihar’s fields still thirsty?

Source: https://news.google.com/rss/articles/CBMihgFBVV95cUxNckxWM05RRFlzRXhKSXotV1U2cHdvWVRfd2ZjcXYxT2lyMTBlb1gzREpnNGFfZUhnTU44QzljNXZvazlwaEFtXzVIRkRRUVcycW1vX28xZUpOUEpyRGpUTWszQTdjeGdKVWRYN0h6cVBnZ2FjTHNLYm5SZFVOWlQtMEdIZGhZUQ?oc=5