Phosphorus (P) is transported into the water resources mainly due to soil erosion. Using the structural equation modeling, this study investigated the relative contribution of different P-producing sources in the Pasikhan River watershed, south of the Caspian Sea, Northern Iran. For this purpose, 79 surface soil samples and 14 suspended sediments were taken from the potential P sources and the river. These sources included undisturbed and degraded rangelands, forests, rice fields, tea gardens, and gullies. The built-in structural equations in PLS software were used for modeling purposes. The overall model fitting index (goodness of fit (GOF) = 0.591) showed the model's strong forecasting capability. Degraded rangelands, gullies, rice fields, and tea gardens significantly contribute to P in the river sediments. In particular, the highest contribution was related to degraded rangelands and gullies (intensity of the effect = 0.63 and 0.47). Finally, the results showed that gullies' contribution was 28.26% to the P production in river sediments, while the other sources had a relatively equal contribution. In conclusion, this study elucidates that undisturbed rangelands and forests exhibit negligible contributions to phosphorus levels in river sediments. Overall, the results confirmed that structural equation modeling is a robust and efficient approach to identifying P sources.
