Precipitation chemistry and nutrient loading by precipitation in a tropical watershed

Abstract

Bulk precipitation samples were taken weekly on the shore of Lake Valencia, Venezuela, over a period of 2 years. All samples were analyzed for total participates, particulate carbon, hydrogen, nitrogen, and phosphorus, major soluble cations and anions, and soluble nitrogen, phosphorus, and carbon compounds. Total insoluble particulate material reaching the watershed averages 139 kg/ha/yr, of which about 25% is organic and 75% inorganic. The total loading rate of insoluble particulates is dramatically higher during the 100‐day dry season than during the remaining portion of the year. The dry season increase in total insoluble particulates is entirely attributable to the inorganic fraction, however, as the particulate carbon and nitrogen loading actually decreases during the dry season. Total loading rates for soluble constituents are generally within the upper range of values reported in the literature for the temperate zone. In terms of equivalents the order of total annual loading rates for soluble cations is Na+ > Mg++ > Ca++ > H+ > NH4+ > K+. For soluble anions the order of loading is HCO3− > Cl− > SO4−− > NO3− > PO43−. Organic nitrogen and organic phosphorus account for major portions of the total nitrogen and phosphorus loading. Soluble silicon loading is quite low. The loading rates of all soluble components on a weekly basis are much higher during the wet season than during the dry season except for soluble inorganic phosphorus, which shows evidence of association with the inorganic particulate fraction that begins to peak during the dry season. There are striking seasonal peaks in the loading rates for almost all precipitation components. Except for soluble inorganic phosphorus and insoluble particulates, which begin to peak well before rains start, the peaks occur very abruptly with the first rains. At this time, as much as 15% of the total annual loading may occur in a flush lasting 1 or 2 weeks. Removal of nitrogen oxides is particularly dramatic and can lead to the development of very acid rainfall at this time (pH < 4.0). Considerable combustion of vegetation occurs toward the end of the dry season and appears to contribute significantly to the buildup of large atmospheric burdens of materials that are removed during the first few rains. A components of variance analysis shows that within the wet season the total amount of rainfall in a given week is the most important variable explaining week‐to‐week variation in loading rates. After the first week or two of flushing there is some continued seasonal decline in loading of some components, but the trend is weak. The amount of insoluble particulate material in a given week during the rainy season accounts for negligible amounts of variance in loading of solubles except for calcium and phosphate, which show suggestive associations with particulates. The radical seasonal changes in loading rates, the flushing effect just at the beginning of the wet season, and the role of combustion in seasonally augmenting loading rates are all factors of possible general importance in the tropics wherever extended dry seasons occur.