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

Table 2

Relationship between breeding industry pollution load and per capita income.

Curve type	Constant term	First-order coefficient	Second-order coefficient	Third-order coefficient	𝑅^2	𝐹
Quadratic coefficient	−11.2718 (−1.86)*	4.5599 (2.79)**	−0.2524 (−2.31)**	—	0.93	114.48
Three coefficients	172.8145 (1.80)*	−69.4635 (−1.80)*	9.6328 (1.87)*	−0.4385 (−1.92)*	0.95	90.34

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