Gather monthly rainfall data, note roof material, and calculate collection efficiency. Metal sheds shed efficiently; rough, thirsty roofs reduce yield. Multiply rainfall depth by roof area and by efficiency to estimate harvest, then plot against seasonal plant demand to reveal gaps, surpluses, and opportunities for reshaping irrigation strategies.
Estimate weekly plant water needs using crop coefficients and evapotranspiration data, then size storage to buffer at least two or three weeks. Oversizing can add cost and stagnation risks; undersizing wastes roof potential. Strive for balance informed by seasons, soil depth, shade, and your maintenance style.