Many things affect water quality. Some of the main ones are temperature, precipitation, soil, and cropping practices. Below, I will describe the effects they have on water quality in a watershed...

Temperature affects biological and chemical activity. At the lower temperatures, plant growth and nutrient up-take are reduced. Therefore, more nutrients in the soil are available for detachment and transport, however, less nutrients will be taken up by plants in the receiving waters. The effect of nutrients on receiving bodies of water during cold weather may be lessened because of the reduced activity of organisms. The hazard to freshwater systems may be even more minimized if the nutrients are transported to the ocean before spring and summer.

Precipitation directly affects the runoff of water and dislodgement and transport of pollutants. Runoff will generally be grater if rainfall is of high intensity or soil moisture levels are high. High-intensity storms increase both detachment, and transport. Raindrop impact on bare soils tends to seal the surface, reducing infiltration and further increasing runoff. Melting snow can also contribute to the runoff-erosion process through shear stresses induced by overland flow. An increase in runoff, especially from rapid snowmelt, provides greater energy to dislodge and transport soil and absorbed substances and increases the volume of water for transporting soluble materials.

A soil's infiltration rate and its ability to absorb pollutants depends in part on its physical, chemical, and biological characteristics. Also, the prior soil moisture content markedly affects the amount of water that can infiltrate. The infiltration rate affects the ration of surface flow to subsurface flow. With an increase in the infiltration rate, the pollutant load associated with surface runoff should decrease. The amount of organic matter and clay particles determines the absorptive capacity of a soil. Sandy soils generally have high infiltration rates and low water holding capacity because of large soil particles and relatively large pores through which water can percolate. Because the total particle surface area and the total negative charge of sandy soils are less, their adsorption capacity is generally much less then that of clay soils. Soils that are both well drained and contain a sufficient amount of clay and organic matter, will absorb the most pollutants.

Cropping practices affect water quality too. Crop cover and surface residue protect the land form the impact of rainfall by absorbing the energy of the raindrops. Close growing crops will generally have less pollution potential then row crops because of the additional canopy. Close-growing vegetation tends to have less pollution potential then row crops because of the additional canopy. Close-growing vegetation tends to reduce soil detachment and filters out suspended organic materials from surface runoff. The longer the growing period of the crop and the ore residue that is left on the field after harvest, the less soil erosion will occur. More often then not, the only pollutants lost by the surface pathway from fields that have excellent cover will be from rotting crop residues. Certain cropping practices can also decrease the pollution potential of a given field by decreasing the need for application