The terms of exchange would be determined through negotiations, but would lie somewhere between the opportunity costs in the two countries. Two of the four cases involve exchanges of rainfed and irrigated output, while two cases involve an exchange of either rainfed or irrigated crops. There is not a unique or standard type of trade that will maximize net welfare. As in any trading opportunity, optimal strategies pertaining to rainfed and irrigated production can be determined only by considering comparative advantages, which are determined by evaluating empirical values of the pertinent opportunity costs. Some authors will expand the scope of their analysis to include additional countries and regions, while others will improve the accuracy of their measurements by collecting more detailed information regarding water use and production methods.