measure the water level in your 'wetland' before remediation (this can be determined post-remediation by finding high-water artifacts on remaining tree stems), and after water levels have stabilized post-remediation. The difference in water levels expressed as a pressure is a direct measure of the change in resistance to flow. If it is possible to find vestiges of the original stream-bed, an absolute measure of return to 'historically normal conditions' can be made.

Plant species transpire differently.  At Coweeta Hydrologic lab in NC, pine transpired about 30inches, hardwoods 20 inches, grasses 10 inches.  In dry areas, plants may have mechanisms like waxy leaves to shut down transpiration under stress.  Recently cleared land will have limited transportation, however invasive plants may return.  There are numerous watershed studies worldwide that document water yield increases with loss of vegetation, conversion, etc.  The density of plants may or may not be a perfect indicator of moisture stress.   Removing invasive species is often short lived unless you treat several times.  The sidebar discussion on water yield may help justify treatment, but it may take many acres of treatment to make a significant difference in streamflow, perhaps 25% of watershed area unless the invasive species directly tap into groundwater.  If the invasive species move seed by water, birds, animals, eradication can be difficult,, costly.  Although I cannot generalize without more information, you may have 3-10 years of water yield increase, dependent on how fast vegetation returns and roots reclaim their former density and moisture stress.  Perhaps herbicide or fire can help hold them back.  As suggested, there may be ways to instrument the site to document change in water table or water stress if that is another goal, comparing a treated area with untreated. ;However, leaving untreated areas of invasive species is risky when you are dealing with invasive species.

My laptop changes words.  transportation should be transpires  in my message.  It might be best to study this on a few sites rather than try to guess the benefits in water yield.  But for example, if you have 500 mm rainfall, and 100 mm water yield, you have 400 mm which is evaporation, transpiration and deep seepage to estimate.  In many instances, deep seepage is small.  From your climate, you may be able to estimate evaporation (remember there is potential evaporation, and actual) at 150 mm, leaving transpirationestimate.  If the transpired estimate was 250 mm, that is the maximum increase on average from clearing the land.  This computer will not let me say transpire the way I want to.  Review literature and see if there are studies for the vegetation transpire, as well as other factors.  Use local data if you have it.

Thanks William,  I totally agree that the differences in transpiration  rates  might have more to do with the water gain after clearing rather than the density. An interesting point I will also look into is  how a one time clearing often has minimal effect than rather when clearing is carried out in repeat treatments.  Additionally while there is quite detailed studies on  the relationship between vegetation cover and water yield, not much has been done for say invasive stands of trees that are in other cases managed as exotic plantations.  I could perhaps use the data we already have on the plantations- water yield relationship  and infer it it to a scenario where the stands are not managed.