There are many things to consider, perhaps too many for a simple reply and a few minutes.  To some degree, the answer will depend on how scarce water is, how important streams and aquatic life are, how many uses, habitats and individuals depend on the waters you might want to divert.  If you owned all the property, the entire watershed, all the resources including the water, and there were no neighbors, fish, animals, communities, farmers or companies that need water downstream, your decision becomes simpler.  If you want to design a system, it is best to know how much water is available, how much can you count on rain, how much you might need to store in case of drought, what types of crops you intend to grow, seasonality of need, what kind of distribution system and cost you are willing to bear.  For these topics and discussions, it becomes not only a hydrology or engineering question, but also a social, environmental and sometimes even a political one between nations, states, communities, neighbors, etc.  

The data on water may or may not be available.  The first thing is to look for rain data, stream flow data in area you are interested in.  Along with that may be who owns the water -- is their water rights legislated or adjudicated?  Knowledge of geology and soils, how much water the soil can hold for use, how much evapotranspiration of vegetation or crops, how much water is runoff that keeps streams alive and properly functioning, and some of which might be stored.  How much water is needed for channel maintenance, so vegetation will not encroach, take it over and cause flood damage due lost channel capacity.  Aquatic biology, stream transport such as boats, recreation, human and animal life may be existing uses of water dependent on available water for drinking and other domestic uses and perhaps other uses that would be damaged if the streams get too low or go dry.  

I suppose there are probably many ways to tackle the mechanics of your question as far as sizing of diversion ditches, irrigation pipes, etc. and those are things that need to be addressed to fit the circumstance.  But diverting water from rivers and streams can have some effects that are not always readily evident.  

I understand there are sometimes environmental tradeoffs necessary for societies and communities to live, and as least then the negative effects will not be surprises, and adjustments can sometimes be made such as storage of water to avoid undue loss of life, resources or property.  

A consideration is also who pays, is it the users of water, is it the irrigation district, state or nation from taxes, or perhaps some other revenues that will be needed to fund design, public interaction, implement work, maintain system, address flood or other damages.  It is probably not unusual to have some degree of civil actions over time due to disagreements or to help cover damages as some will gain, some will loose, and it may take work to make it fair.  That is often why various ways have been used to identify who has water rights, what amount and under what conditions can water be used.  Also there may be impacts to landowners and properties where ditches and canals would be located.  Will the system be open all the time, seasonal or just on certain days to help ration water?

So before designing the network of diversion ditches, pipelines and canals, there may be substantial more prework than actually the hydrologic and engineering calculations of the irrigation system features.  In most instances, an integrated resource and technical approach and possibly with some public involvement would yield the most satisfying result and avoid problems.  

There are a variety of equations and papers, tables that discuss flow in pipes, ditches, culverts that are available on the internet.  Most will require things like ditch or canal cross section, gradient, roughness, head of water level differences at inlet and outlet. Would it be lined to limit infiltration?  More papers and models at estimating water yield, water balances, etc. to help estimate supply of water.