There is just too much uncertainty in long term seasonal rain fall levels, especially with the effects of climate change as well as El Niño and La Niña cycles. Resulting in low production years.
[1]http://www.sandiegouniontribune.com/business/energy-green/sd... [2] https://www.theguardian.com/world/2017/dec/08/malawi-blackou... [3] https://www.theguardian.com/news/2016/mar/30/weatherwatch-ha...