Environmental Kuznets Curve Analysis of the Economic Development and Nonpoint Source Pollution in the Ningxia Yellow River Irrigation Districts in China

Table 1

Relationship between agricultural pollution load and per capita income.

Curve type	Constant term	First-order coefficient	Second-order coefficient	Third-order coefficient	𝑅^2	𝐹
Quadratic coefficient	−18.4389 (−2.43)**	7.6465 (3.74)***	−0.5115 (−3.74)***	—	0.47	6.99
Three coefficients	35.0630 (0.26)	−13.8673 (−0.26)	2.3615 (0.33)	−0.1274 (−0.40)	0.47	4.48

Note. ***, **, and *, respectively, represent significance levels in 1%, 5%, and 10%. The 𝑡-test results are written in brackets.