You have many options. What ...

You have many options. What you are describing is TIERED agricultural. (steps). This type is most popular in china. See if this fits your plans.  Build a retaining wall at the end of each step. Strong enough to hold back the water.  If you are growing rice allow the rain to flood the field. Select a bottom feeding fish and place them into the water. Fish will supply oxygen so will not get stagnate. When you harvest rice harvest also. Still need compost. For reserve rainwater higher up the hill find and dig out a large rainwater pond. If too much soil line with bricks/mortar add mineral rocks. descending canal to contiguous step fields. Easy to open and close flow gates.  If evaporation concern the your canal should be a special design. I will try to type it.  Standard canals look like this:   I_______I.    low evaporation canals look like this:  /_______\     Base is 3 feet and top is 1 foot.  Your design 6 feet base and 6 inch open at top. You decide.   Chinese also chop the top off rock mountains bevel it out for a few hundred hectares catch the rain, large step fields, farmers build a home there, tend to field daily.  So many options that are inexpensive and simple.  Just need to learn natures rules and all will become clear.