Determinants of water quality in agricultural watersheds

Water q u a l i t y was moni tored f o r 17 months i n s i x d i f f e r e n t a g r i c u l t u r a l watersheds t o eva lua te t h e impact o f a1 t e r a t i o n s i r~ r i p a r i a n vege ta t i on and channel morphology. Sampling was conducted d u r i n g bo th base f l o w and r u n o f f per iods . Dur ing base f lows, where r i p a r i a n vege ta t i on had been removed, i n t e r a c t i o n s between b i o l o g i c a l processes o f ins t ream o rgan i c p roduc t i on and hyd ro log i ca l processes o f seasona l l y low f lows a r e t h e p r imary determinants of seasonal dynamics of suspended s o l i d s , t u r b i d i t y and p a r t i c u l a t e phosphorus concent ra t ions . Peak l e v e l s o f a1 1 parameters a r e reached d u r i n g t h e summer p e r i o d o f increased b i o t i c a c t i v i t y . Ins t ream organ ic p roduc t i on i s o f l e s s importance i n r e g u l a t i n g water q u a l i t y where r i p a r i a n vege ta t i on i s mainta ined and f l o w s a r e n o t i n t e r m i t t e n t . Concentrat ions of suspended s o l i d s remain r e l a t i v e l y constant , and seasonal dynamics of phosphorus and t u r b i d i t y appear t o be c o n t r o l l e d by l e a f i n p u t s i n f a l l . I n t e r m i t t e n t f l o w c o n d i t i o n s i n summer enhance t h e importance o f ins t rearn o rgan i c p roduc t i on i n c o n t r o l l i n g water q u a l i t y , even when r i p a r i a n vege ta t i on i s present . Dur ing r u n o f f events, a s imp le model based on t h e KLS f a c t o r s o f t h e Un iversa l S o i l Loss Equat ion a c c u r a t e l y p r e d i c t s s p a t i a l p a t t e r n s o f suspended so l i ds , t u r b i d i t y , and phosphoru's l e v e l s i n watersheds w i t h a un i f o rm channel ized stream, w e l l p ro tec ted stream banks, and c u l t i v a t i o n t o t h e stream edge. The model was n o t an accura te p r e d i c t o r where he te rogene i t y i n r i pa r i an vege ta t i on and channel morphology e x i s ted . Suspended s o l i d s concen t ra t ions were g r e a t e r t han p r e d i c t e d i n areas where uns tab le ( f i n e ) substates occur and p o t e n t i a l energy o f t h e stream i s h i g h due t o removal o f r i p a r i a n vege ta t i on and c r e a t i o n o f a u n i f o r m and s t r a i g h t channel. T iming o f t h e peak concen t ra t i ons i n these areas seemed t o be r e l a t e d t o ma jo r f l ushes o f d ischarge due t o r a p i d urban dra inage o r de layed a d d i t i o n o f su r f ace o r subsur face i n p u t s . T h i s suggests t h a t water q u a l i t y d u r i n g r u n o f f events i s p r i m a r i l y governed by h y d r o l o g i c a l processes. B i o l o g i c a l processes determine i n c i p i e n t cond i t i ons . The hydro1 o g i c a l processes i n v o l ve complex i n t e r a c t i o n s between watershed e ros ion p o t e n t i a l , channel e q u i l i b r i u m , hyd ro log i ca l impacts of r i p a r i a n vege ta t ion , and magnitude o f t h e r u n o f f event. These r e s u l t s i n d i c a t e t h a t t h e r e l e v a n t t h e o r y f o r enhancement o f water resources and f o r mode l l i ng water q u a l i t y i n a g r i c u l t u r a l watersheds v a r i e s depending on f l o w cond i t i ons . Dur ing base f low, emphasis should be p laced on l i n k i n g h y d r o l o g i c a l t heo ry o f ins t ream t r a n s p o r t of i n o r g a n i c m a t e r i a l t o b i o l o g i c a l t heo ry o f p roduc t i on and t r a n s p o r t of o rgan ic m a t e r i a l . Dur ing r u n o f f events, emphasis should be p laced on l i n k i n g o f e ros ion p r e d i c t i o n s f rom a g r i c u l t u r a l t heo ry t o geomorphological t h e o r y o f strearn e q u i l i b r i u m and sediment t r a n s p o r t . Such i n t e r d i s c i p l i n a r y l i nkages should p rov ide resource management dec i s i ons which w i l l have t h e g r e a t e s t chance of success and broadest range o f a p p l i c a b i l i t y . Schlosser, Isaac J., and Karr, James R. , DETERMINANTS OF WATER QUALITY I N AGRICULTURAL WATERSHEDS, UNIVERSITY OF ILLINOIS WATER RESOURCES CENTER RESEARCH REPORT NO. 147 F i n a l r e p o r t t o t h e Water Resources Center, U n i v e r s i t y o f I 1 1 i n o i s , Urbana , 1980.