Last update: November 24, 2010 08:57:33 AM E-mail Print

 

MODELLING NAMA-KAROO SUBSHRUB PHYTOMASS PRODUCTION


PCV du Toit

Grootfontein Agricultural Development Institute, Private Bag X 529, MIDDELBURG, 5900

 


The monotonous subshrub vegetation seen in the Nama-Karoo depends for production on mean climatic variables. A model is now proposed to predict aboveground available phytomass production from mean climatic inputs. Monthly aboveground available phytomass production for a number of subshrub species were measured. These figures were correlated with mean climatic data recorded during the same period. Winter, (May, June, July, August), 50% of the rainfall contributes to the predicted production; in autumn; (March, April), and spring (September, October), 50% of the current month's rainfall plus 50% of the previous month's rainfall contributes to the predicted production. Summer, (November, December, January, February), 50% of the current month's rainfall plus 50% of the previous month's rainfall, plus 25% of the rainfall received two months earlier, contributes to the predicted production. This rain value is multiplied by 3; the production estimate in grammes per mm of rain received in savanna grassland (R). Mean temperature is: {maxT - 0.25 (maxT - minT)} (T). Mean daily values per month of sunlight hours are calculated (S). Mean evaporation per month is calculated (E). Mean wind run per month is calculated (W). Predicted values are multiplied by two, to evaluate measured and prediction values:

The production prediction model = {([S + T]*R) + (W*E)}*2.

Aboveground available phytomass production of Nama-Karoo subshrubs can be modelled and these values can be used to calculate grazing capacities. The predicted production curve has a correlation coefficient of 0.8130 with the actual measured production curve.