The discharge rate from solar powered pump depends on the radiation and not on the temperatures. In fact, the power generated from solar panels decreases (slightly) with increase in ambient temperature and thus, water outflow from a solar powered pump at higher ambient temperature will be slightly lesser than at a lower ambient temperature for the same level of incoming solar radiation on the solar panels. 

It should be noted that in winters, the cold temperature is often associated with reduced availability of sunshine and hence, the output of the solar powered pump is likely to be reduced in winter when compared to summer. This is not due to the reduced temperature but due to fact that the solar radiation falling on the solar array is reduced