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Knowledge about long-term average flow is essential for planning and managing water resources because it represents the potential water availability. One technique used to determine streamflow is regionalization, but because most gauge stations normally are associated with large drainage areas, the extrapolation of regionalization equations does not accurately represent the water availability; therefore, this method is not recommended. The main objective of the present paper is to propose a new method of estimating water availability that minimizes the risks of extrapolating regionalization equations for long-term average flow. The method is based on the use of a threshold value of the runoff coefficient to obtain the long-term average flow at the positions of the basin where the runoff coefficient estimated by the regionalization equation exceeds the threshold value. It was found that values of the runoff coefficient estimated average flows greater than twice those of the threshold values. The use of this method allows a reliable estimation for long-term average flows in regions of extrapolation without compromising the security of the available water supply.

Hydrologic modeling is a t ool used to better under- stand and represent the hydrologic behavior of a hydro- graphic basin as the use of hydrologic model shows great potential to characterize the water availability under cli- mate or land use condition changes. The objective of the present work was to develop a model to estimate the mini- mum streamflow of the Paracatu river and its tributaries based on precipitation data. For this study data from 21 stream and 30 rain gages were analyzed. The development of the model to estimate the minimum streamflow from precipitation data was base d on the baseflow recession curve, identifying the parameters that define the 21 stream gages, and these were the recession coefficient of the baseflow ( α ) and the groundwater flow into the main channel at the beginning of the recession (Q0). The results obtained al- lowed to conclude that: the total precipitation in the months that preceded the Q0 occurrence allowed a good estimation of the behavior of this groundwater flow; and the greatest deviations obtained for the values of α occurred in the stream gages located in headwater regions where the topo- graphic, climatic and land use conditions tend to show greater discrepancies in relation to the average conditions of the basin

A major effort has been dedicated to the reconstitution of natural discharges. The natural discharge is the sum of the consumed discharges and the observed ones. The effect of anthropic actions in the Paracatu basin increased water consumption about 11 times in the last 27 years. This study was undertaken to evaluate the impact of the use of natural discharges to substitute the observed ones in different sections of Paracatu basin. The data of the observed and natural discharges of 21 rivergaging stations from 1976 to 2000 were analyzed. The impact of the use of the natural discharges was estimated to the maximum (Qmax), the medium of long duration (Qmld) and the minimums discharges (Q7,10; Q95 and Q90) by using an index, characterized in this study as ∆ (delta). The correlation between the drainage area of each section and its respective delta was calculated. The delta behavior was analyzed using a graphic, according to the values of the natural discharges. The statistical analysis used the Student t test (α = 5%). The mean value of the ∆ for Qmax, Qmld ,Q90, Q95 and Q7,10 was, respectively, 0.01%, 0.66%, 4.26%, 5.45% and 5.76%. All these deltas were considered significant (α = 5%). The highest values of discharges led to the lowest values of delta. The deltas of the maximum, medium of long duration and minimums discharges were statistically different. The correlation between the drainage area and the delta was not significant. The delta behavior according to the values of the natural discharges followed a potential model with negative declivity at a high R2 (0.9842). For the purpose of hydrologic studies of the Paracatu basin, the use of natural discharges in the case of the maximum and medium of long duration is not a concern, but in the case of the minimums discharges, it is recommended to take some precautions