The crops with the most demand for water are in the polytunnels, the one place in the garden where it doesn't rain, of course. To address this, and to reduce the amount of soggy ground around the polytunnels (we grow crops up close to them) we have designed and built our own water collection and pumping system. It is simple, so not much to go wrong, and has worked very well for the last two years.
Collection tanks partially buried to enable both collection from inline gutter and from tanks on other side of polytunnels (which are set higher up to allow flow into tanks you can see in photo).
Simple petrol pump sends water up to header tanks at the top of the polytunnels. 1000 litres takes about 4 minutes. We tried a solar powered pump system - it wasn't powerful or fast enough. On a smaller polytunnel/greenhouse the solar system would work fine.
Mini guttering is fixed with brackets to the timber side rail (part of the side ventilation system). To direct the water off the polytunnel membrane and into the guttering, we have stuck a line of sticky repair tape (duct tape would work as well) onto the membrane. The tape is folded onto itself where it makes contact with the gutter, so that the gutter can be removed for the winter to avoid inevitable snow damage.
Header tanks: The polytunnels are sited on a slope so we can use gravity from the raised tanks to trickle the water through an irrigation pipe back down the incline. The yellow hose connected to the tap is atteched to the required irrigation line(s) once in the polytunnel.
Makeshift filter made from offcut of enviromesh to catch leaves, insects etc. Needs cleaning after every downpour, but easily done.